JPH08213676A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE: To realize gain flattening, regarding an input signal optical power in a wide wavelength range and further in a wide dynamic range. CONSTITUTION: An optical fiber amplifier 1 is concerned wherein one or more amplification parts 2 and 3 which parts have optical fibers 23 and 32 for amplification, are cascaded respectively. A filter part 5 whose cutoff wavelength is variable and filter control parts 4, 6 which change the cutoff wavelength of the filter according to the input signal optical power are installed, in at least one or more parts on the input side of a first stage amplification part, the interstage of amplification parts, and the output side of a final stage amplification part. As a pumping light source in at least one or more amplification parts, power variable pumping light sources capable of changing the pumping light power are used, and the following may be installed; a pumping light control part which changes the pumping light power from each of the power variable light sources according to the input signal optical power, and one or two or more fixed filter parts which are set on the signal light path and have fixed cutoff wavelengths.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier, and is particularly suitable for application to an optical fiber amplifier (EDFA) using an optical fiber (EDF) having Er ³⁺ (erbium ion) added to a core. It is a thing.

[0002]

2. Description of the Related Art Heretofore, many optical repeaters have been used in which input signal light is converted into an electric signal, amplified, and then converted into an optical signal for transmission. However, the configuration of such an optical repeater is large, and therefore, an optical repeater equipped with an EDFA for performing direct optical amplification is being actively developed and researched, and is entering the practical stage.

In a conventional EDFA, one end of an incident side optical fiber and one end of an exit side optical fiber are connected to both ends of an EDF (amplifying optical fiber) via an isolator, and an excitation light source is connected to the EDF. The emitted excitation light is made incident through the optical coupler. The excitation light emitted from the excitation light source and incident on the EDF is
It can be absorbed by the EDF and cause a sufficient population inversion. When the input signal light is incident on the EDF from the incident side optical fiber while the pump light is incident on the EDF, the input signal light is gradually amplified by the stimulated emission action of the EDF, and the amplified signal light is emitted. It is transmitted as output signal light through the side optical fiber.

As described above, the EDF can directly amplify the signal light by using the stimulated emission effect. Here, the signal light wavelength is often selected to be a wavelength band of 1550 to 1560 nm, which is a wavelength band in which the loss in the silica optical fiber is the smallest. As the wavelength of the excitation light, the 980 nm band or the 1480 nm band, which is the absorption band or absorption line of EDF, is used.

An optical fiber amplifier (EDFA) in which amplifier units having an EDFA structure are connected in multiple stages (for example, two stages) has already been proposed.

[0006]

The EDFA is 1530
Since there is an amplification region in the nm to 1560 band, it is effective for a narrow band optical transmission system with a single wavelength. However, in order to effectively use the laid optical fiber transmission line, a wavelength division multiplexing (WDM) system in which multiple wavelength channels are collectively transmitted is preferable.

Here, when the EDFA is applied to the transmission system adopting the WDM system, improvement of the wavelength-gain deviation characteristic (gain flatness of each wavelength) is an important issue. That is, in the case where there are many relay stages using EDFA, if the gain in each wavelength channel in the EDFA is not flat, the signal power deviation between each wavelength after transmission becomes considerably large, and the maximum relay transmission distance becomes the minimum amplitude. This is because the S / N ratio of the wavelength signal is limited. Therefore, flattening the gain of the EDFA is an indispensable technique for suppressing variations in signal power and S / N ratio between wavelength channels.

A number of gain flattening methods for EDFAs have already been proposed. For example, a method of equalizing the gain dependence of wavelength using an optical wavelength filter, a method of adding high-concentration Al (aluminum) to adjust the gain characteristic of EDF,
It is known that a method of cooling the EDF at an extremely low temperature is effective.

However, when these methods are used, gain flattening can be achieved in the 1540 to 1560 nm band, but it cannot be applied to a wider band. In addition, in practice, the distance between repeaters is not constant, and the signal light power input to the EDFA is also not constant, which also makes it difficult to flatten the gain over a wide band.

FIG. 2 is a characteristic diagram for explaining such a problem, and is a characteristic diagram of wavelength vs. gain deviation when Al is added to the EDF to achieve gain flattening.

The characteristic curve indicated by the solid line in FIG.
It shows the wavelength-gain deviation characteristic of the one-stage EDFA to which the EDF added by weight% is applied.
Gain deviation at 1560nm is small, but 1540nm
There is a drop in gain in the band and a gain peak in the 1530 nm band. The broken line characteristic curve in FIG. 2 shows the wavelength vs. gain deviation characteristic of the EDFA having a one-stage structure to which the EDF containing 5% by weight of Al is applied.
The gain deviation at 60 nm is small and the band is wide, but
There is a gain peak in the 1530 nm band, and 1530 to 156
Gain flattening in the 0 nm band (wide band) has not been realized.

In FIG. 2, the input signal light power Pin is -25d.
In the case of Bm, although not shown (see FIG. 5 described later), as the input signal light power Pin increases, the magnitude of the gain peak from the flat gain decreases and the input signal light power Pin increases considerably. Then, gain saturation occurs in the 1530 nm band, and the input signal light power Pin is -25 dB.
The characteristic curve is different from the case of m.

Therefore, there is a demand for an optical fiber amplifier which can realize gain flattening for a wide wavelength range and a wide range of input signal light power.

[0014]

In order to solve the above problems, the present invention provides an optical fiber amplifier in which an amplifier section having an amplification optical fiber therein is cascade-connected in N (N is 1 or more) stages, It is characterized in that a gain smoothing means for smoothing the gain according to the input signal light power is provided.

The present inventor has
It was found that the flatness of the gain characteristic is different, and therefore, the optical fiber amplifier is provided with a gain smoothing means for smoothing the gain according to the input signal light power.

Here, as the gain smoothing means, there are cutoff wavelengths provided at least at one or more positions on the input side of the first stage amplifier section, between the stages between the amplifier sections, and on the output side of the final stage amplifier section. It is preferable to include a variable filter unit and a filter control unit that varies the cutoff wavelength of each variable filter unit according to the input signal light power.

In this optical fiber amplifier, the gain of a band having a gain peak different from the flat gain in the desired wavelength range in the conventional optical fiber amplifier is suppressed by the filter means having a variable cutoff wavelength, and the gain is suppressed. Since the gain peak amount changes depending on the input signal light power, the filter control means changes the cutoff wavelength of the filter means according to the input signal light power.

Further, as the gain smoothing means, it is a pumping light source in at least one or more amplifier units, and a power variable pumping light source capable of varying the pumping light power, and each power variable pumping according to the input signal light power. It is preferable to include a pumping light controller that varies the power of the pumping light from the light source, and one or more fixed filter units that are provided on the path of the signal light and have a fixed cutoff wavelength.

In this optical fiber amplifier, the pumping light controller varies the gain deviation of the band having a gain peak different from the flat gain in the desired wavelength band in the conventional optical fiber amplifier according to the input signal light power. By controlling the pumping light power from the pumping light source, it is made constant regardless of the magnitude of the input signal light power, and this gain deviation is removed by a fixed filter means with a fixed cutoff wavelength, which results in a wide range. Gain flattening is achieved for a wide range of input signal light power in the wavelength range.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

(A) First Embodiment Hereinafter, a first embodiment in which the present invention is applied to an EDFA will be described with reference to the drawings. Here, FIG. 1 shows this first
2 shows the overall configuration of the embodiment of FIG.

In FIG. 1, the ED of the first embodiment is shown.
The FA 1 is based on a two-stage configuration in which a preamplifier unit (front stage optical fiber amplifier unit) 2 and a postamplifier unit (post stage optical fiber amplifier unit) 3 are connected in cascade.

In the case of the first embodiment, the preamplifier unit 2 adopting the forward excitation method is applied. In the preamplifier section 2, on the path of the signal light, from the incident side, the isolator 21 and the optical coupler 2 having wavelength selectivity are provided.
2, the EDF 23 to which 5 wt% of Al is added, and the isolator 24 are sequentially arranged.
A pumping light source 25 made of, for example, a laser diode is connected to the pumping light source 25.
(80 nm band; 1480 nm band may be used) is supplied to the EDF 23 via the optical coupler 22.

On the other hand, the post-amplifier section 3 adopts the backward pumping method. In the post-amplifier section 3, an isolator 31, an EDF 32 containing 5% by weight of Al, an optical coupler 33 having wavelength selectivity, and an isolator 34 are sequentially arranged on the signal light path from the incident side. A pumping light source 35 formed of, for example, a laser diode is connected to the optical coupler 33,
Excitation light from the excitation light source 35 (for example, 980 nm band; 1
480 nm band may be used) E through the optical coupler 33
It is supplied to the DF32.

Therefore, the amplifying action of the preamplifier section 2 and the postamplifier section 3 alone is well known. That is, the excitation light emitted from the excitation light source 25 or 35 and incident on the EDF 23 or 32 via the optical coupler 22 or 33 is absorbed by the EDF 23 or 32 to cause a sufficient population inversion, and the excitation light is the EDF 23 or When the signal light is incident on the EDF 23 or 32 while being incident on 32, the signal light is gradually amplified by the stimulated emission action of the EDF 23 or 32, and the amplified signal light is sent out as outgoing signal light. It Note that the isolators 21 and 24 or 31 and 34 prevent the amplifying operation in the preamplifier unit 2 or the postamplifier unit 3 from being adversely affected by others, and prevent the amplifying action from affecting others. It is provided for.

In the first embodiment, in addition to the basic structure that has existed in the conventional EDFA as described above, the optical coupler 4 and the long wavelength by a dielectric multilayer film whose cutoff wavelength can be adjusted are used. Pass filter (LWPF) 5,
And LWPF for controlling the cutoff wavelength of LWPF 5
A control unit 6 is provided.

The optical coupler 4 is provided at the input stage of the EDFA 1 and splits the wavelength-multiplexed input signal light from the incident side optical fiber into two and makes one branch signal light incident on the preamplifier section 2. Together with the other branch signal light
The light is incident on the WPF controller 6.

The LWPF 5 is inserted on the signal light path between the preamplifier unit 2 and the postamplifier unit 3, and cuts the short wavelength side of the signal light amplified by the preamplifier unit 2 and has a cutoff wavelength. Is controlled by the LWPF controller 6. Here, although the cutoff wavelength is variable, it generally changes in the vicinity of 1530 nm.

The LWPF control section 6 detects the power of the input signal light branched by the optical coupler 4 and changes the cutoff wavelength of the LWPF 5 according to the detected power. Here, the cutoff wavelength is shifted to the shorter wavelength side as the input signal light power is higher, and the gain in the 1530 nm band is changed.

Various existing structures can be applied to the LWPF 5 having variable wavelength characteristics.
Two examples will be briefly described.

FIG. 3 shows the above-mentioned LWP having variable wavelength characteristics.
It shows a first configuration example of F5.

This LWPF 5 is an LWPF plate (LWP) made of a dielectric multilayer film whose transmission center wavelength is uniform regardless of location.
F main body) 51a, and a stepping motor 52 whose rotation shaft is attached to the LWPF plate 51a so as to change the angle of the LWPF plate 51a in response to a control signal from the LWPF control unit 6 and the LWPF plate 51a. It is composed of collimator lenses 53 and 54 provided. One collimator lens 53 is provided at the end of the optical fiber 53a that propagates the output signal light from the isolator 24, and the other collimator lens 54
Is provided at the end of the optical fiber 54a that propagates the input signal light to the isolator 34.

The wavelength of the signal light that has propagated through the optical fiber 53a and is emitted from the collimator lens 53 as parallel light into the space has its short wavelength component cut by passing through the LWPF plate 51a, and this short wavelength component is cut. The signal light is collected by the collimator lens 54 and introduced into the optical fiber 54a.

Here, the LWPF in the direction orthogonal to the optical axis
If the inclination angle θ of the plate 51a is 0, the LWPF plate 51a
The optical path that transmits light becomes shortest and the central wavelength becomes long wavelength. As the tilt angle θ increases, the LWPF plate 51a
As the optical path length of light passing through the center wavelength becomes longer, the center wavelength shifts to the short wavelength side, and the cutoff wavelength also shifts as the center wavelength shifts. That is, the LWPF controller 6 determines the stepping motor 52 according to the detected power of the input signal light.
Via, the inclination angle θ of the LWPF plate 51a is controlled,
The cutoff wavelength of LWPF5 is changed.

FIG. 4 shows the above-mentioned LWP having variable wavelength characteristics.
It shows a second configuration example of F5.

The LWPF 5 includes an LWPF plate (LWPF body) 51b made of a dielectric multilayer film whose transmission center wavelengths are continuously different from each other in the sliding direction, and an LWP.
A stepping motor 52 that rotates by an angle according to a control signal from the F control unit 6, and a rotation / linear motion conversion mechanism (for example, a rack and a pinion) 5 that slides the LWPF plate 51b according to the rotation direction and amount of the stepping motor 52.
5 and collimator lenses 53 and 54 provided to face each other with the LWPF plate 51a interposed therebetween.

Also in this configuration example, the optical fiber 53a
The wavelength of the signal light that has propagated through and is emitted into the space as parallel light from the collimator lens 53 has its short wavelength component cut by passing through the LWPF plate 51b, and the signal light with this short wavelength component cut is the collimator lens. The light is condensed by 54 and introduced into the optical fiber 54a.

Here, since the transmission center wavelength of the LWPF plate 51b is continuously different in the sliding direction, the center wavelength of the signal light transmitted through the LWPF plate 51b depends on the sliding position of the LWPF plate 51b during irradiation. Changes, and the cutoff wavelength also shifts according to this center wavelength. That is, the LWPF control unit 6 controls the slide position of the LWPF plate 51b via the stepping motor 52 and the rotation / linear motion converting mechanism 55 in accordance with the detected power of the input signal light to set the cutoff wavelength of the LWPF 5. Change.

In the case of the first configuration example of the LWPF 5,
Polarization Dependent Loss (PDL) increases as the transmission center wavelength (and thus the cutoff wavelength) shifts toward the shorter wavelength side (the tilt angle θ increases).
However, in the second configuration example, even if the transmission center wavelength (and thus the cutoff wavelength) is shifted to the short wavelength side, the PD
L is uniform. However, there are more components in the second configuration example than in the rotation / linear motion converting mechanism 55.

The first embodiment is 1530 to 1560n.
In the m band, the configuration is as shown in FIG. 1 with the intention that the gain flattening can be achieved regardless of the input signal light power.

The concept of adopting such a configuration will be described below with reference to FIGS. 2 and 5 above. It should be noted that FIG. 5 shows the configuration of the optical coupler 4 according to the first embodiment.
6 shows input signal light power versus gain characteristics for four wavelengths in a configuration (conventional EDFA having a two-stage configuration) excluding the LWPF 5 and the LWPF control unit 6.

First, let us consider a configuration excluding the optical coupler 4, the LWPF 5, and the LWPF control unit 6 from the configuration of the first embodiment. As shown in FIG. 2 described above, A
1-stage ED to which EDF containing 5% by weight of 1 is applied
The FA has a small gain deviation in the wavelength range of 1540 to 1560 nm, and can achieve the gain flatness in this wavelength range.
The same applies to the stage configuration. Moreover, as is clear from the input signal light power-gain characteristics for the four wavelengths in FIG. 5, the gain flatness can be achieved in the wavelength range 1540 to 1560 nm over a wide range of the input signal light power Pin.

Therefore, in the first embodiment, EDF containing 5% by weight of Al is used as the EDF 23 and EDF 32, and the wavelength range of 1540 to 1560 nm is used.
Gain flatness at Therefore, 153
The same gain may be obtained in the 0 (up to 1540 nm) band.

Al is used as EDF 23 and EDF 32.
When the EDF added with weight% is applied, as shown in FIG. 2, it has a gain peak in the 1530 nm band.
Gain peak wavelength range 1540 to 1 in the 530 nm band
As shown in FIG. 5, the deviation from the flat gain of 560 nm becomes smaller as the input signal light power Pin becomes larger, and when the input signal light power Pin becomes considerably larger, the gain falls to the other wavelength band or more ( (Due to gain saturation).

Therefore, it is necessary to suppress the gain peak in the 1530 nm band and avoid gain saturation in the 1530 nm band.

Since the power Pin of the input signal light to the EDFA is not unique due to the loss of the optical fiber on the input side, it is not applicable to suppress the gain peak in the 1530 nm band with a fixed wavelength filter. Further, even if there is a filter having the inverse characteristic of the wavelength vs. gain characteristic for a certain input signal light power Pin, it is difficult to obtain a filter having the inverse characteristic over a wide range of the input signal light power Pin. Therefore, in this embodiment, the LWPF 5 whose cutoff wavelength can be adjusted is provided, and the cutoff wavelength of the LWPF 5 is controlled according to the input signal light power Pin, so as to adjust the cutoff wavelength according to the input signal light power Pin 1530.
It was decided to change the amount of suppression of the gain peak in the nm band. Therefore, the LWPF 5 is a pseudo inverse characteristic filter of the wavelength vs. gain characteristic for a wide range of power Pin of the input signal light.

Here, the LWPF 5 is inserted between the preamplifier section 2 and the postamplifier section 3 at 1530 nm.
It also considers avoiding gain saturation in the band.

The LWPF 5 is inserted at (1) the front stage of the preamplifier unit 2, (2) between the preamplifier unit 2 and the postamplifier unit 3, and (3) the rear stage of the postamplifier unit 3 (this case will be described later). Other embodiments are thus configured).

When the LWPF 5 is inserted before the preamplifier section 2 (1), A in the EDF 23 of the preamplifier section 2 is set.
Since the noise component due to SE is also amplified by the post-amplifier unit 3, it is not preferable in terms of noise figure (NF). In addition, when the variable LWPF is further provided on the subsequent stage side, it may be provided at this position.

When the LWPF 5 is inserted after the post-amplifier section 3 (3), the amplification of the pre-amplifier section 2 causes
Post-amplifier section 3 for 1530 nm band with gain peak
Is further amplified, so 1530n before filtering
There is a risk of gain saturation in the m band. The ED is compensated for the input signal light power Pin being small to some extent.
In FA, such an insertion position may be used, which is another embodiment of the present invention. Experimentally, the gain flatness is inferior to the case of (2) according to the first embodiment.

When the LWPF 5 is inserted between the preamplifier section 2 and the postamplifier section 3 as in the first embodiment, the noise component due to ASE in the EDF 23 of the preamplifier section 2 can be removed by filtering. Since the gain peak in the 1530 nm band is suppressed before being input to the unit 3, such an insertion position is preferable for the reason that gain saturation does not occur even when amplification is performed in the post amplifier unit 3.

Although the noise component generated by the post-amplifier unit 3 cannot be removed, the signal light having a considerable power and amplified by the pre-amplifier unit 2 and having a small noise component is input to the post-amplifier unit 3. Even if noise is mixed in the post-amplifier section 3, a sufficient S / N ratio can be secured, and there is no practical problem.

In the first embodiment, it goes without saying that the wavelength range of the output signal light from the post-amplifier section 3 is 1530-.
It suffices if the gain flatness can be secured at 1560 nm. Therefore, at the signal light stage from LWPF5, 1530 nm
The gain of the band does not have to be the same as the gain of the other wavelength bands 1540 to 1560 nm, and the gain in the signal light stage from the LWPF 5 is the wavelength band of the output signal light after the amplification processing of the post-amplifier unit 1530 to 1560 nm. It suffices that the gain flatness is ensured at, and in accordance with this, the LWPF5
The cutoff wavelength of is selected.

In the conventional two-stage EDFA in which the preamplifier section and the postamplifier section are connected in cascade, one isolator is provided between the preamplifier section and the postamplifier section. However, in this embodiment, L
Since the WPF 5 is inserted between the preamplifier unit 2 and the postamplifier unit 3, the reflection at the LWPF 5 is caused by the preamplifier unit 2
And so as not to affect any of the post amplifier section 3,
Isolators 24 and 31 are provided before and after the LWPF 5.

FIG. 6 shows the experimental results of the input signal light power vs. gain characteristics of four wavelengths in the configuration of the first embodiment. As is clear from the comparison between FIG. 5 and FIG. 6, in the EDFA 1 of the first embodiment, the gain of the signal light output from the postamplifier unit 3 is in the wavelength range 1530 over a wide range of the input signal light power Pin. ~ 1560nm
Has been flattened by.

Therefore, according to the first embodiment, the LWPF 5 capable of varying the cutoff wavelength is used as the preamplifier section 2.
And the post-amplifier unit 3 so that the cutoff wavelength can be varied according to the input signal light power.
It is possible to realize an EDFA which can achieve gain flattening over a wide range of input signal light power and also over a wide wavelength range.

(B) Second Embodiment FIG. 7 shows a second embodiment of the present invention. Corresponding to FIG. 1 described above, the same parts are designated by the same reference numerals. is there.

The EDFA 1-2 of the second embodiment is
Not only the LWPF 5 having a variable cutoff wavelength for suppressing the gain peak in the 1530 nm band is inserted between the preamplifier unit 2 and the postamplifier unit 3, but also the second LWP
The F5-2 is provided on the output side of the post-amplifier section 3, and in order to comply with this installation, the second branch optical coupler 4-2 and the second branch optical coupler 4-2 are provided.
The LWPF control unit 6-2 (which may share the LWPF control unit 6) is provided.

That is, the suppression function according to the input signal light power of the gain peak in the 1530 nm band is used for two LWPs.
This is realized by F5 and 5-2.

Therefore, according to the second embodiment as well,
It is possible to realize an EDFA which can achieve gain flattening over a wide range of input signal light power and also over a wide wavelength range. In addition, according to the second embodiment, the noise component generated by the post-amplifier unit 3 is converted into the second LWPF5-
2 can be removed.

(C) Third Embodiment FIG. 8 shows a third embodiment of the present invention. Corresponding to FIG. 7 described above, the same parts are designated by the same reference numerals. is there.

The EDFA 1-3 of the third embodiment is
The installation position of the second branching optical coupler 4-2 in the second embodiment is the output position of the LWPF 5.
Since the power of the signal light output from the LWPF 5 depends on the power of the input signal light, the cutoff wavelength of the second LWPF 5-2 is variable according to the power of the input signal light also in the third embodiment. Has been done.

Therefore, according to the third embodiment as well,
It is possible to realize an EDFA which can achieve gain flattening over a wide range of input signal light power and also over a wide wavelength range. In addition, according to the third embodiment, the noise component generated by the post-amplifier unit 3 is reduced to the second LWPF5-.
2 can be removed.

(D) Fourth Embodiment Next, a fourth embodiment of the present invention applied to an EDFA will be described with reference to the drawings. Here, FIG. 9 shows this fourth
1 shows the overall configuration of the embodiment of FIG.
The same or corresponding portions as and are designated by the same reference numerals.

In FIG. 9, the ED of the fourth embodiment is shown.
FA1-4 also has a preamplifier unit (pre-stage optical fiber amplifier unit)
2 and a post-amplifier section (post-stage optical fiber amplifying section) 3 are basically connected in a two-stage configuration.

Also in the case of the fourth embodiment, the preamplifier unit 2 adopting the forward pumping method is applied, and the preamplifier unit 2 includes an isolator 21 and an optical coupler 2.
2, EDF 23 to which 5% by weight of Al is added, an isolator 24, and a pumping light source 25.

On the other hand, the post-amplifier unit 3 adopts the backward pumping method. In the post-amplifier section 3, an EDF 32 containing 5 wt% of Al, an optical coupler 33 having wavelength selectivity, and an isolator 34 are sequentially arranged on the path of the signal light from the incident side.
A pumping light source 36, which is, for example, a laser diode, is connected to the optical coupler 33, and pumping light (eg, 980 nm band; 1480 nm band) from the pumping light source 36 is supplied to the EDF 32 via the optical coupler 33. To be done.

In the case of the fourth embodiment, since the LWPF 5 is not provided between the preamplifier section 2 and the postamplifier section 3, only one isolator (24) is provided between the preamplifier section 2 and the postamplifier section 3. Is enough.

Since the internal configurations of the preamplifier section 2 and the postamplifier section 3 are as described above, the amplifying action of the preamplifier section 2 and the postamplifier section 3 alone is well known. That is, it is emitted from the excitation light source 25 or 36,
The pumping light that has entered the EDF 23 or 32 via the optical coupler 22 or 33 is absorbed by the EDF 23 or 32 to cause a sufficient population inversion, and the pumping light is input to the EDF 23 or 32. When entering the EDF 23 or 32, the signal light is gradually amplified by the stimulated emission action of the EDF 23 or 32, and the amplified signal light is sent out as outgoing signal light. The isolators 21, 24, and 34 are provided to prevent the amplifying operation in the preamplifier unit 2 or the postamplifier unit 3 from being adversely affected by others, and to prevent the amplifying action from affecting others. ing.

In the fourth embodiment, in addition to the basic structure that has existed in the conventional EDFA as described above, the optical coupler 4 has a long cut-off wavelength, for example, a long wavelength formed by a dielectric multilayer film. Pass filter (fixed LWPF)
7 and the excitation light source controller 8 are provided. Further, as the excitation light source 36 in the post-amplifier section 3,
The one that can change the excitation light power is applied.

The optical coupler 4 is provided at the input stage of the EDFA 1-4, and splits the wavelength-multiplexed input signal light from the incident side optical fiber into two, and branches one of the branched signal light into the preamplifier section 2. In addition to being incident, the other branched signal light is incident on the excitation light source controller 8.

The fixed LWPF 7 is provided on the output side of the post-amplifier section 3 and cuts the short wavelength side of the output signal light from the post-amplifier section 3. Here, the cutoff wavelength is selected in the vicinity of 1530 nm.

The pumping light source controller 8 detects the power of the input signal light branched by the optical coupler 4 and varies the pumping light power from the power variable pumping light source 36 according to the detected power. is there. Here, the larger the input signal light power, the larger the pumping light power.

In the fourth embodiment, 1530 to 156 are provided.
In the 0 nm band, it is configured as described above with the intention of achieving gain flattening regardless of the input signal light power. Hereinafter, the concept of adopting such a configuration will be described with reference to the drawings.

First, let us consider a configuration excluding the optical coupler 4, the LWPF 7, and the pumping light source control section 8 from the configuration of the fourth embodiment. In this case, it is possible to say the same as when the characteristic configuration is excluded from the configuration of the first embodiment. That is, as shown in FIG. 2 described above, the single-stage EDFA to which the EDF containing 5% by weight of Al is applied has a small gain deviation in the wavelength range of 1540 to 1560 nm and achieves the gain flatness in this wavelength range. It is possible to achieve the same with the two-stage configuration. Moreover, as is clear from the above-mentioned input signal light power vs. gain characteristic of FIG. 5, in the wavelength range 1540 to 1560 nm, the gain flatness can be achieved over a wide range of the input signal light power Pin.

Therefore, also in the fourth embodiment, the EDF in which 5 wt% of Al is added is applied as the EDF 23 and the EDF 32, and the wavelength range 1540 to 1560 nm.
Gain flatness at Therefore, 153
The same gain may be obtained in the 0 (up to 1540 nm) band.

Al is used as EDF 23 and EDF 32.
When the EDF added with weight% is applied, as shown in FIG. 2, it has a gain peak in the 1530 nm band.
Gain peak wavelength range 1540 to 1 in the 530 nm band
As shown in FIG. 5, the deviation from the flat gain of 560 nm becomes smaller as the input signal light power Pin becomes larger, and when the input signal light power Pin becomes considerably larger, the gain falls to the other wavelength band or more ( (Due to gain saturation). Therefore, the wavelength range 1540 to 156
When the flat gain at 0 nm and the gain in the 1530 (to 1540 nm) band are made the same, suppression of the gain peak in the 1530 nm band and avoidance of gain saturation in the 1530 nm band are required.

FIG. 10 is a virtual characteristic diagram for explaining the processing image in the fourth embodiment. Flat gain in the wavelength range of 1540 to 1560 nm and 1530 nm
When the band gains are made the same, the wavelength range 1540 as shown in FIG. 10A (similar to FIG. 5 but reprinted) is shown.
The situation where the deviation between the flat gain at ˜1560 nm and the gain in the 1530 nm band changes depending on the input signal light power Pin is set to a state where the gain deviation is constant as shown in FIG. 10B. With this configuration, the fixed gain deviation can be removed by the fixed filter.

It should be noted that the constant gain deviation means that in the region where the input signal light power Pin is large, it is 15 compared to the conventional case.
This means that the gain in the 30 nm band is raised, and the gain saturation is also reduced.

In the fourth embodiment, as a means for keeping the gain deviation constant regardless of the input signal light power, the pumping light power from the power variable pumping light source 36 is changed according to the input signal light power Pin. The fixed LWPF 7 is provided as means for removing the constant gain deviation.

Next, by varying the pumping light power from the variable power pumping light source 36 according to the input signal light power Pin, a characteristic with a constant gain deviation as shown in FIG. 10B can be obtained. Will be described with reference to FIG.

FIG. 11A shows the input signal light power Pin and the output light from the postamplifier section 3 when the pumping light power of the preamplifier section 2 is 50 mW and "the pumping light power of the postamplifier section 3 is 50 mW". 4 shows the relationship with the gain for the four wavelengths. On the other hand, FIG. 11B shows a case where the pumping light power of the preamplifier unit 2 is 50 mW and the pumping light power of the postamplifier unit 3 is 100 mW.
4 shows the relationship between the input signal light power Pin and the gain of the output light from the post-amplifier section 3 for four wavelengths.

Here, the wavelength band 1540 in the case where the input signal light power Pin in FIG. 11A relating to “the pumping light power of the post-amplifier unit 3 is 50 mW” is −40 dBm is now assumed.
Gain deviation Δ in the 1530 nm band with respect to ˜1560 nm
Focus on G. “The pumping light power of the post-amplifier unit 3 is 1
In FIG. 11B relating to “00 mW”, this gain deviation ΔG is obtained when the input signal light power Pin is approximately −22 dBm. Therefore, conversely, when the input signal light power Pin is -40 dBm, the pumping light power of the post-amplifier unit 3 is set to "50 mW", and the input signal light power Pin
Is -22 dBm, if the pumping light power of the post-amplifier unit 3 is set to "100 mW", the wavelength range 1540 to 156
It was found that the gain deviation ΔG in the 1530 nm band with respect to 0 nm can be made the same, and similarly when the input signal light power Pin is other, the pumping light power in the post-amplifier unit 3 is appropriately selected to obtain 1 for the wavelength range 1540 to 1560 nm.
It can be seen that the gain deviation ΔG in the 530 nm band can be made the same.

FIG. 12 shows the input signal light power P when the pump light power of the post-amplifier unit 3 is changed stepwise.
7 shows an experimental result of the relationship between in and the gain of the output light from the post-amplifier unit 3 for four wavelengths. FIG.
The signal light from the post-amplifier unit 3 is further fixed to LWP.
It shows the experimental result of the relationship between the gain for four wavelengths of the signal light after passing through F7 and the input signal light power Pin. As is clear from comparison with FIG. 5, the wavelength range 1540
It can be seen that the gain deviation in the 1530 nm band with respect to ˜1560 nm can be made constant by the output light from the post-amplifier unit 3, and it can be removed by the fixed LWPF 7.

Therefore, according to the fourth embodiment, the fixed LWPF 7 having a fixed cutoff wavelength is provided, and
Since the pumping light power of the post-amplifier unit 3 is made variable according to the input signal light power, it is possible to realize an EDFA that can achieve gain flattening over a wide range of the input signal light power and in a wide wavelength range. it can.

(E) Fifth Embodiment FIG. 14 shows a fifth embodiment of the present invention.
Corresponding parts to those in FIGS. 1 and 9 described above, the same parts are designated by the same reference numerals.

The EDFAs 1-4 of the fifth embodiment are
Both the characteristic configuration of the EDFA 1 of the first embodiment and the characteristic configuration of the EDFA 1-3 of the fourth embodiment are provided. That is, the optical couplers 4 and 4-2 and the variable LWPF
5, an LWPF control unit 6, a fixed LPWF 7, and a pumping light source control unit 8.

As is clear from the description of the first embodiment, the variable LWPFs 5 and LW are also provided in the fifth embodiment.
Since the PF control unit 6 is provided, the gain in the 1530 nm band can be obtained in the wavelength range 1540 to 156 by these configurations.
The flat gain at 0 nm can be matched. Also, the fourth
As is clear from the description of the embodiment, the pump light source 36 having the variable pump power and the fixed LW are also provided in the fifth embodiment.
Since the PF 7 and the pumping light source control unit 8 are provided, the gain in the 1530 nm band can be obtained in the wavelength range 1 by these configurations.
It is possible to obtain a flat gain at 540 to 1560 nm.

As described above, in the fifth embodiment, two types of gain flattening configurations are used together so that the flattening error in each type is mutually compensated.

Therefore, according to the fifth embodiment, the fixed LWPF 7 having a fixed cutoff wavelength is provided, the pumping light power of the postamplifier 3 is changed according to the input signal light power, and the cutoff wavelength is changed. Variable L that can change
Since the WPF 5 is interposed between the preamplifier section 2 and the postamplifier section 3 so that the cutoff wavelength can be varied according to the input signal light power, the gain can be obtained over a wide range of the input signal light power and in a wide wavelength range. An EDFA that can achieve flattening can be realized.

(F) Other Embodiments (F-1) In each of the above embodiments, the preamplifier unit 2 is considered to be suitable as a combination of two stages of amplifier units.
Has shown that the forward pumping method is adopted and the post-amplifier section 3 adopts the backward pumping method, the preamplifier section 2 may adopt the backward pumping method or the bidirectional pumping method. The part 3 may employ a forward excitation method or a bidirectional excitation method.

(F-2) In each of the above embodiments, the amplifier units are cascaded in two stages, but the present invention can be applied to one stage or three or more stages in cascade connection.

In this case, the variable LWPF may be arranged as follows. In the case of one stage, an LWPF having a variable cutoff wavelength may be arranged on the output side. Further, in the case of three or more stages, the LWPF having a variable cutoff wavelength may be arranged at least at one or more positions on the input side of the first stage amplifier section, between the stages, and on the output side of the final stage amplifier section. It should be noted that when one LWPF is provided, it should be avoided to provide it on the input side of the first-stage amplifier section for the above-mentioned reason. However, when two or more LWPFs are provided, one of them is provided as the first-stage amplifier. It may be provided on the input side of the amplifier section.

(F-3) As a modified embodiment of the fourth embodiment, the fixed LWPF 7 is provided between the preamplifier section 2 and the postamplifier section 3, and the fixed LWPF 7 is provided on the input side of the preamplifier section 2. be able to. Also, a plurality of fixed L
The thing which provided WPF7 can be mentioned. That is, in the fourth embodiment, the gain deviation is made constant and then the gain deviation is removed. However, after the gain in the 1530 nm band is attenuated in advance by a certain gain deviation, the post-amplifier 3 The pumping light power may be adjusted so that its gain is matched with the wavelength range 1540 to 1560 nm.

(F-4) As a modified embodiment of the fourth embodiment, not only the pumping light power of the postamplifier section 3 is adjusted but also the pumping light power of the preamplifier section 2 is adjusted. 1 for the wavelength range 1540 to 1560 nm
There may be mentioned one that makes the gain deviation constant in the 530 nm band, and one that makes the gain deviation constant in the 1530 nm band by changing only the pumping light power of the preamplifier section 2.

(F-5) As a modified embodiment of the fifth embodiment, there can be mentioned an arbitrary combination of two types of gain flattening structures.

For example, gain flattening is performed by adjusting the pumping light power of the preamplifier unit 2 (in this case, fixed LWP).
F7 is, for example, the input side of the preamplifier unit 2), and then a variable L provided between the preamplifier unit 2 and the postamplifier unit 3.
Gain flattening may be performed by changing the cutoff wavelength by the WPF 5. Further, for example, the gain is flattened by adjusting the pumping light power of the preamplifier unit 2 (in this case, the fixed LWPF 7 is, for example, the input side of the preamplifier unit 2), and then provided between the preamplifier unit 2 and the postamplifier unit 3. The gain may be flattened by changing the cutoff wavelength by the variable LWPF 5, and further the gain may be flattened by adjusting the pumping light power of the postamplifier 3.

(F-6) As a modified embodiment of the fifth embodiment, the fixed LWPF 7 that functions in gain flattening by adjusting the pumping light power of the post-amplifier section 3 is omitted, and the function of the fixed LWPF 7 is omitted. The variable LWPF 5 can also be used.

(F-7) In each of the above embodiments, the EDFA
However, the present invention can be applied to optical fiber amplifiers to which other rare earth elements are added. For example, praseodymium-doped optical fiber amplifiers and neodymium-doped optical fiber amplifiers are in the stage of further research and evaluation in terms of characteristics.
Although there are many unclear points as to whether they have the same wavelength-to-gain deviation characteristic or input signal light power-to-gain characteristic as those of A, the present invention can be applied if the same characteristics are exhibited. In this case, if a band having a gain peak different from the flat gain appears in the long wavelength side in the desired wavelength range, a short wavelength pass filter with a variable cutoff wavelength may be applied.

[0099]

As described above, according to the optical fiber amplifier of the present invention, the cutoff wavelength of the variable filter section inserted on the signal path can be varied according to the input signal light power, and / or Since the gain smoothing means for varying the pumping light power according to the input signal light power and fixedly attenuating the signal light power is provided, it is possible to realize gain flattening over a wide wavelength range and a wide range of input power. become.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment.

FIG. 2 is a wavelength vs. gain deviation characteristic diagram of a conventional one-stage optical fiber amplifier.

FIG. 3 is a block diagram showing a first configuration example of a variable LWPF of the first embodiment.

FIG. 4 is a block diagram showing a second configuration example of the variable LWPF of the first embodiment.

FIG. 5 is an input signal light power vs. gain characteristic diagram for four wavelengths of a conventional optical fiber amplifier having a two-stage configuration.

FIG. 6 is an input signal light power versus gain characteristic diagram for four wavelengths in the first embodiment.

FIG. 7 is a block diagram showing a configuration of a second exemplary embodiment.

FIG. 8 is a block diagram showing a configuration of a third exemplary embodiment.

FIG. 9 is a block diagram showing a configuration of a fourth embodiment.

FIG. 10 is a model characteristic diagram illustrating the operating principle of the fourth embodiment.

FIG. 11 is a characteristic diagram for explaining the method of making the gain deviation of the fourth embodiment independent of the input signal light power.

FIG. 12 is an input signal light power versus gain characteristic diagram for four wavelengths of output light of the post-amplifier unit 3 of the fourth embodiment.

FIG. 13 is an input signal light power vs. gain characteristic diagram for four wavelengths of the output light of the fixed LWPF 7 of the fourth embodiment.

FIG. 14 is a block diagram showing a configuration of a fifth embodiment.

[Explanation of symbols]

1, 1-2, 1-3, 1-4, 1-5 ... EDFA (optical fiber amplifier), 2 ... Preamplifier section, 3 ... Postamplifier section, 4, 4-2 ... Branching optical coupler, 5, 5 -2 ... Variable LWPF (variable filter part), 6, 6-2 ... LWPF control part (filter control part), 7 ... Fixed LWPF (fixed filter part), 8 ... Excitation light source control part (excitation light control part), 23, 32 ... EDF (amplifying optical fiber), 36 ... Power variable pumping light source.

[Procedure amendment]

[Submission date] November 21, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction content]

This LWPF 5 is an LWPF plate (LWP) made of a dielectric multilayer film whose transmission center wavelength is uniform regardless of location.
F main body) 51a, and a stepping motor 52 whose rotation shaft is attached to the LWPF plate 51a so as to change the angle of the LWPF plate 51a in response to a control signal from the LWPF control unit 6 and the LWPF plate 51a. It is composed of collimator lenses 53 and 54 provided. One collimator lens 53 is provided at the end of the optical fiber 53a that propagates the output signal light from the isolator 24, and the other collimator lens 54
Is provided at the end of the optical fiber 54a that propagates the input signal light to the isolator 31 .

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0035

[Correction method] Change

[Correction content]

The LWPF 5 includes an LWPF plate (LWPF body) 51b made of a dielectric multilayer film whose transmission center wavelengths are continuously different from each other in the sliding direction, and an LWP.
A stepping motor 52 that rotates by an angle according to a control signal from the F control unit 6, and a rotation / linear motion conversion mechanism (for example, a rack and a pinion) 5 that slides the LWPF plate 51b according to the rotation direction and amount of the stepping motor 52.
5 and collimator lenses 53 and 54 which are provided to face each other with the LWPF plate 51b interposed therebetween.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0045

[Correction method] Change

[Correction content]

Since the power Pin of the input signal light to the EDFA is not unique due to the loss of the optical fiber on the input side, it is not applicable to suppress the gain peak in the 1530 nm band with a fixed wavelength filter. Further, even if there is a filter having an inverse characteristic of the wavelength vs. gain characteristic with respect to a certain input signal light power Pin, it is difficult to obtain a filter having an inverse characteristic that changes over a wide range of the input signal light power Pin. is there. Therefore, in this embodiment, the LWPF 5 whose cutoff wavelength can be adjusted is provided, and the cutoff wavelength of the LWPF 5 is controlled according to the input signal light power Pin, so that the gain peak in the 1530 nm band according to the input signal power Pin. It was decided to change the amount of suppression of. Therefore, the LWPF 5 is a pseudo inverse characteristic filter of the wavelength vs. gain characteristic for a wide range of power Pin of the input signal light.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0048

[Correction method] Change

[Correction content]

When the LWPF 5 is inserted before the preamplifier section 2 (1), A in the EDF 23 of the preamplifier section 2 is set.
Since noise components due to SE increase , noise figure (NF)
It is not preferable from the viewpoint of. In addition, further variable LW on the rear side
When the PF is provided, it may be provided at this position.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0050

[Correction method] Change

[Correction content]

When the LWPF 5 is inserted between the preamplifier unit 2 and the postamplifier unit 3 as in the first embodiment, the noise component due to ASE in the EDF 23 of the preamplifier unit 2 can be removed by filtering. Since the gain peak in the 1530 nm band is suppressed before inputting to the unit 3, even by amplification in the post-amplifier unit 3,
Such an insertion position is preferable because it is difficult to cause gain saturation .

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shusei Aoki 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (5)

[Claims] 1. An optical fiber amplifier in which N (N is 1 or more) stages of amplifier units having an amplifying optical fiber therein are cascaded, and a gain smoothing means for smoothing a gain according to an input signal light power. An optical fiber amplifier characterized by being provided. 2. As the gain smoothing means, cutoff wavelengths provided at least at one or more positions on the input side of the first-stage amplifier section, between the stages between the amplifier sections, and on the output side of the final-stage amplifier section are provided. The optical fiber amplifier according to claim 1, further comprising a variable filter section and a filter control section that varies a cutoff wavelength of each variable filter section according to input signal light power. 3. The amplifying optical fiber is an erbium-doped optical fiber, and the amplifier unit is connected in two stages, and the variable filter unit formed of a long wavelength pass filter is arranged between the stages. Item 3. The optical fiber amplifier according to Item 2. 4. The gain smoothing means is a pumping light source in at least one or more amplifier sections, the power variable pumping light source capable of varying pumping light power, and each of the powers according to the input signal light power. A pumping light controller that changes the power of the pumping light from the variable pumping light source, and a fixed cutoff wavelength provided on the signal light path.
The optical fiber amplifier according to claim 1, further comprising two or more fixed filter units. 5. The amplifying optical fiber is an erbium-doped optical fiber, the amplifier section is connected in two stages, and the fixed filter section consisting of a long wavelength pass filter is arranged on the output side of the subsequent stage amplifier section, and The optical fiber amplifier according to claim 4, wherein a power variable pumping light source capable of varying pumping light power is applied as the pumping light source in the latter-stage amplifier section.