JP2002319726A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently amplify a C band and an L band. SOLUTION: A C-band signal light SC and an L-band signal light SL are input to an input terminal 10. An EDF 16 is excited strongly by an excitation light 20a from an LD 20, the signal light SC is amplified, a C-band ASE light is generated, and the signal light SL is slightly amplified. An optical circulator 18 outputs the light from the EDF 16 to the fiber grating 22 of 90% C-band reflection from a port B. The reflected light of the grating 22 is incident to fiber grating 32 of 100% C-band reflection via the optical circulator 18, an optical fiber 36 and an optical circulator 34, is reflected here, and is arrived at an output terminal 39 via the ports B, C of the circulator 34. An EDF 26 amplifies the signal light SL by the C-band light passed through the grating 22 and an excitation light 38a. The amplified signal light SL reaches an output terminal 39 via a WDM optical coupler 38, an optical isolator 30, the grating 32 and the circulator 34.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier,
More specifically, the present invention relates to an optical amplifier that amplifies two bands such as a C band and an L band.

[0002]

2. Description of the Related Art In the field of wavelength division multiplexing optical transmission, the use of an L band (1.57 to 1.60 .mu.m band) in addition to a conventional C band (1.52 to 1.57 .mu.m band) is being studied. . As such a broadband optical amplifier, a configuration in which a C-band optical amplifier and an L-band optical amplifier are arranged in parallel is described in US Pat. That is, in the broadband optical amplifier, the incident light is divided into the C band and the L band by the optical circulator and the reflector of the C band, and each is separately amplified, and then combined by the optical circulator and the reflector of the C band. . Also, a configuration is known in which a demultiplexing filter that separates the C band and the L band is used instead of the optical circulator and the reflector.

Further, an erbium-doped optical fiber (EDF) for amplifying the C band, a 100% reflector for the C band, an EDF for amplifying the L band, and a 100% reflector for the C band and the L band.
% Reflectors are serially arranged so that the C-band signal light reciprocates the C-band amplification EDF, and the L-band signal light reciprocates the C-band amplification EDF and the L-band amplification EDF. The configuration is also known (JP-A-20
00-58953).

At present, the configuration of an L-band optical amplifier generally used is as follows, except that the EDF length is as long as one digit.
Same as the C-band optical amplifier. That is, the L-band optical amplifier has a longer EDF, 1480 than the C-band.
and a WDM coupler for multiplexing pump light, and an optical isolator for preventing return light.

[0005] Initially, an L-band optical amplifier using a 1550 nm band LD as an excitation light source was proposed (for example, JF Massicott, J. Arm).
itage, R.E. Wyatt, B .; J. Ai
nslie and S.L. P. Craig-Rya
n, "High gain, broadband,
1.6 μm Er ³⁺ doped Silica f
iber amplifier, Electron.
Lett. , 1990, 26, no. 20,1645
-1646).

In order to improve the noise figure, 155
A configuration that excites at two wavelengths of 0 nm and 1480 nm has also been proposed (for example, JF Mascotot,
R. Wyatt, and B.W. J. Ainsli
e, "Low noiseoperation of
Er ³⁺ doped Silica fiberam
qualifier around 1.6 μm ”, Ele
ctron. Lett. , 1992, 28, no. 2
0, 1924-1925).

An L-band optical amplifier using a 1550 nm band LD has the following problems. That is, 1550
nm pumping has higher gain efficiency than 1480 nm or 980 nm pumping, but has a problem of a large noise figure.
In terms of cost and reliability, the 1480 nm or 980 nm band L widely used as an excitation light source for a C-band optical amplifier is used.
It is more advantageous to use D. From these viewpoints, 1550
An L-band optical amplifier using an nm-band LD as an excitation light source is not used at present.

[0008] With the two-wavelength pumping configuration, the noise figure can be improved to the same extent as with the 1480 nm pumping. However, since a 1550 nm LD, which is not commercially available, is generally required and two types of LDs are used, the structure is complicated and the cost is high. It has not been.

An experiment of two-wavelength excitation has been performed on a quartz EDF (hereinafter, referred to as EDSF), but not on a fluoride EDF (hereinafter, referred to as EDFF).
For the fluoride EDF, see H.E. Ono, M .; Y
amada and Y. Ohshi, "Gain-
flattened Er ³⁺ -doped five
r amplifier for a WDM sig
nal in the 1.57-1.60 μm w
average region ”, IEEE Ph
otonics Techn. Lett. , 199
7, 9, No. 5,596-598; On
o, M. Yamada T. Kanamiri,
S. Sudo and Y. Ohshi, "1.
58 μm ban fluoride-based Er
^{3 +} -doped fiber amplifier f
or WDM transmission system
ms ", Electron. Lett., 1997,
33, no. 17, 1471-1472.

[0010] It has been reported that when using an EDFF, the band is expanded by 5 nm to 1565 to 1600 nm (H. Ono, M. Yamada, TK).
anamori, S .; Sudo and Y. O
hishi, "1.58 μm band fluorid
e-based Er ³⁺ -doped fiber
amplifier for WDM transmi
session systems ", Electron.
Lett. , 1997, 33, no. 17,1471-
1472).

It has been reported that the 980 nm pump improved the noise figure by about 0.3 dB from the 1480 nm pump, and as a result realized a noise figure of 5.0 dB (HO).
no, M .; Yamada, S.M. Sudo an
d Y. Ohshi, “1.58 μm band
Er ³⁺ -doped fiber amplifier
r pumped in the 0.98 and
1.48 μm bands ”, Electron. L
ett. , 1997, 33, no. 10,876-87
7).

There has also been proposed a configuration in which two-stage optical amplifiers are connected in series to reduce noise and increase the bandwidth. For example,
EDSF of 980 nm pump was used as the optical amplifier at the front stage, and EDSF of 1480 nm pump was used as the optical amplifier at the subsequent stage.
, A gain of 32.8 dB and a noise figure of 5.
0 dB and an amplification band of 30 mn are realized. In the case where 980 nm pumped EDSF is used as the first stage optical amplifier and 1480 nm pumped EDFF is used as the second stage optical amplifier, a gain of 25.0, a noise figure of 5.1 dB and an amplification band of 35 nm are realized. Also, in the case where 980 nm pumped EDSF is used as the first optical amplifier and 1480 nm pumped EDSF is used as the second optical amplifier, the length of each EDSF is optimally adjusted so that the gain is 20 dB, the noise figure is 5 dB, and Amplification band 70
nm broadband amplifier has been obtained. For example, M. Y
amada, H .; Ono and Y. Ohish
i, "low-noise, broadband E
r ³⁺ -doped silica fiber am
privifiers ", Electron. let
t. , 1998, 34, no. 15, 1490-149
See No. 1.

Further, two EDs are connected via an optical isolator.
SFs are connected and the two EDSFs are bidirectionally pumped to obtain a gain of 30 dB, a noise figure of 6 dB or more, and an amplification band of 47 nm (H. Sawada, M.S.
Yoshida, K .; Imamura and
Y. Imada, "Broadband and g
ain-flattened erbiumu-dop
ed fiber amplifier with +
20 dBm output power for 1
580 nm band amplifiatio
n ", ECOC'99, 26-30 Septembe
r, 1999, Nice, France).

[0014]

In a conventional example in which a C-band optical amplification path and an L-band optical amplification path are separately provided, not only many optical elements are required, but also the size of the apparatus becomes large. This is because although there is a difference between the C band and the L band, it is necessary to arrange optical amplifiers having the same configuration for each optical amplification path.

In principle, the EDF for L band amplification is C
It needs to be three to four times longer than the EDF for band amplification. That is, in the EDF for L band amplification, 0.9
This is because the excitation light in the 8 μm or 1.48 μm band excites the C band, the C band excites the L band, and as a result, the L band signal light has a two-stage excitation structure. EDF has a nonlinear effect due to its structure,
Since it has a high value for both chromatic dispersion and polarization mode dispersion, it is a major factor that deteriorates transmission characteristics.
In this regard, it is desirable that the EDF be as short as possible. However, in a conventional example in which a C-band optical amplification path and an L-band optical amplification path are separately provided, it is difficult to shorten the optical amplification fiber. Also, it is difficult to improve the excitation efficiency.

EDF for C band amplification and E for L band amplification
In a conventional example in which DFs are serially arranged and the C band and the L band are optically amplified by the reflection method, the excitation light source can be shared by the C band optical amplification fiber and the L band optical amplification fiber.
However, since there are two reflectors on one line and a plurality of connection points, laser oscillation is likely to occur due to a round trip between them. This means that it is difficult to increase the gain. In other words, a one-pass type optical amplifier in which signal light travels in only one direction has difficulty in laser oscillation even when the gain is increased, and has high reliability.

In the conventional reflection type, both the C-band signal light and the L-band signal light can be efficiently amplified with a small number of components. However, the L-band optical amplifier has a smaller gain and a larger noise figure than the C-band optical amplifier. For practical use, it is desirable to increase the pumping efficiency and gain, widen the amplification band, and reduce the noise figure.

An object of the present invention is to provide a broadband optical amplifier which can be realized with a smaller number of components.

Another object of the present invention is to provide an optical amplifier capable of amplifying the C band and the L band with higher efficiency.

Another object of the present invention is to provide a high gain, low noise, wide band optical amplifier having a wide amplification band.

[0021]

An optical amplifier according to the present invention receives a signal light including a first band signal light and a second band signal light, and amplifies the first band signal light.
Of the light output from the first optical amplification medium and most of the light of the first band is supplied to the first optical path, and the rest is supplied to the second optical path. An optical amplifier, a second optical amplifying medium on the second optical path for amplifying the signal light of the second band, light propagating through the first optical path, and second light The second amplified by the amplification medium
And a multiplexer for multiplexing the signal light of the band and supplying the multiplexed signal light to the output terminal.

With such a configuration, the first optical amplifying medium amplifies the first band light with low noise. The demultiplexer is
A part of the first band light output from the first optical amplification medium is supplied to a second optical amplification medium. The second optical amplifying medium uses the part of the first band light as the auxiliary light to form the second band.
Band signal light is amplified with low noise. In this way,
The signal light of the first band and the signal light of the second band can be amplified with low noise.

Preferably, the demultiplexer is connected to an input terminal of the second optical path and reflects the light of the first band with a reflectance of less than 100%; A first light for transferring light output from the optical amplification medium to the first reflector and transferring light from the first reflector to the first optical path;
An optical circulator. Thus, with a simple configuration, of the light output from the first optical amplification medium, most of the light of the first band can be supplied to the first optical path, and the rest is supplied to the second optical path. it can.

Preferably, the multiplexer is connected to an output end of the second optical path and substantially couples the light of the first band to one.
A second reflector that reflects 00% and a second optical circulator that transfers light from the first light path to the second reflector and transfers light from the second reflector to the output terminal. Have. Thus, the amplified first and second band signal lights can be combined with a simple configuration.

The optical amplifier according to the present invention further includes a first pumping light supply device for supplying a first pumping light to the first optical amplifying medium. The first excitation light is preferably supplied to the first optical amplification medium from the front. Thus, the first band signal light can be amplified with low noise.

Preferably, the first pumping light supply device outputs the first pumping light with such an optical power that the first pumping light exists over the entire length of the first optical amplification medium. . Thus, the first optical amplifying medium transmits the signal light of the second band with low loss or positive gain.

Preferably, the first optical amplifying medium has a positive gain with respect to the second band (L) under the first pump light. Thereby, low noise and wide band can be realized with the gain characteristic of the second band as a whole.

[0028] The optical amplifier according to the present invention further comprises a second pumping light supply device for supplying the second pumping light to the second optical amplifying medium. Thereby, many signal lights of the second band can be amplified.

When the first optical path includes an optical transmission medium for compensating for the delay of the second optical path in the second band, a second band component passing through the first path exists. However, good amplification characteristics of the second band can be obtained.

For example, the first band is the C band, and the second band is the L band. In this case, the second
Preferably, the optical amplification medium is made of a fluoride fiber. Thereby, a high gain of L-band amplification can be obtained with a short optical fiber.

The optical amplifier according to the present invention also receives a signal light including a first band signal light and a second band signal light,
A first optical amplification medium for amplifying the signal light of the first band, and first, second, third, and fourth ports, wherein input light of the first port is output from the second port, and An optical circulator that outputs input light of a port from a third port and outputs input light of the third port from a fourth port, wherein light from the first optical amplification medium is input to the first port. A circulator, an optical transmission line having a second optical amplifying medium for amplifying the signal light of the second band, and an optical transmission line disposed between the optical transmission line and a second port of the optical circulator; Is disposed between the optical transmission path and the third port of the optical circulator, and the signal light of the second band is substantially 100% reflective. And a second reflector that reflects light at

With such a configuration, the first optical amplifying medium optically amplifies the first band light with low noise, and the second optical amplifying medium uses a part of the first band light as auxiliary light. By doing so, the signal light of the second band is amplified with low noise. As a result, the first band signal light and the second band signal light can be amplified with low noise.

Preferably, the optical circulator has first, second, and third ports, and outputs input light of the first port from the second port, and outputs input light of the second port from the third port. And a second port of the second optical circulator, a first port of the second optical circulator is a first port of the optical circulator, and a second port of the second optical circulator is a second port of the optical circulator. A third port of the second optical circulator is connected to a first port of the third optical circulator, a second port of the third optical circulator is a third port of the optical circulator, and a third port of the third optical circulator is provided. The third port is the fourth port of the optical circulator.

The optical amplifier according to the present invention is further disposed between the third port of the second optical circulator and the first port of the third optical circulator, and the optical transmission line in the second band (L). And an optical transmission medium (36) for compensating for the delay. Thereby, even when the second band component is transmitted between the third port of the second optical circulator and the first port of the third optical circulator,
Good amplification characteristics of the second band can be obtained.

The optical amplifier according to the present invention further includes a first pumping light supply device for supplying the first pumping light to the first optical amplifying medium. The first excitation light is preferably supplied to the first optical amplification medium from the front. Thus, the first band signal light can be amplified with low noise.

Preferably, the first pumping light supply device outputs the first pumping light with such an optical power that the first pumping light exists over the entire length of the first optical amplification medium. . Thus, the first optical amplifying medium transmits the signal light of the second band with low loss or positive gain.

Preferably, the first optical amplification medium has a positive gain with respect to the second band (L) under the first pump light. Thereby, low noise and wide band can be realized with the gain characteristic of the second band as a whole.

Preferably, the first reflector is provided with the first reflector.
Of the excitation light. Thereby, the remainder of the first pump light can be used for pumping the second optical amplification medium, and the overall pump efficiency is improved.

The optical amplifier according to the present invention further comprises a second pumping light supply device for supplying a second pumping light to the second optical amplifying medium. Thereby, many signal lights of the second band can be amplified.

For example, the first band is the C band, and the second band is the L band. In this case, the second
Preferably, the optical amplification medium is made of a fluoride fiber. Thereby, a high gain of L-band amplification can be obtained with a short optical fiber.

[0041]

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

FIG. 1 shows a schematic block diagram of an embodiment of the present invention. Signal light S _L of the signal light S _C and L-band of C-band to be the optical amplification is input to the input terminal 10. The input terminal 10 is a WDM optical coupler 12, an optical isolator 1
4. Erbium-doped optical fiber (ED
F) Connect to port A of optical circulator 18 via 16. The optical circulator 18 is an optical element that has three ports A, B, and C, outputs input light of port A from port B, and outputs input light of port B from port C.

The optical isolator 14 is connected to the EDF 16 through the W
They are arranged in a direction that blocks light traveling toward the DM optical coupler 12.

The laser diode (LD) 20 is an EDF
The excitation light 20 a for amplifying the C-band light by exciting the excitation light 16 is generated and supplied to the WDM optical coupler 12. EDF16
, The length and erbium doped amount is adjusted to be suitable for amplifying a C-band signal light S _C.

The port B of the optical circulator 18 is provided with a fiber grating 22, an optical isolator 24, and an L band amplifying E which reflect the C band with a reflectance of less than 100%.
DF26, WDM optical coupler 28, optical isolator 30
And an optical circulator 34 via a fiber grating 32 having a reflectance of 100% for reflecting the C-band signal light.
To port B of The optical circulator 34 is an optical element having the same function as the optical circulator 18. A fiber 36 for phase adjustment is connected between the port C of the optical circulator 18 and the port A of the optical circulator 34.

The optical isolators 24 and 30 are connected to the port B of the optical circulator 34 in order to prevent unnecessary laser oscillation.
Are arranged in such a direction as to block light in a direction from the optical circulator 18 to the port B.

The laser diode (LD) 38 is an EDF
An excitation light 38 a for exciting the L-band light by exciting 26 is generated and supplied to the WDM optical coupler 28. The WDM optical coupler 28 supplies the pump light 38a from the LD 38 to the EDF 26 in a direction opposite to the propagation direction of the signal light SL. E
DF26, the length and erbium doped amount is adjusted to be suitable for amplifying the L-band signal light S _L.

The port C of the optical circulator 34 is connected to the output terminal 39. The signal light S amplified by the present embodiment
_{C, S L,} the port C of the optical circulator 34, i.e.,
Output from the output terminal 39.

The output power of the laser diode 20 is E
It is set so large that the DF 16 cannot be absorbed, in other words, the excitation light 20a exists with sufficient intensity over the entire area of the EDF 16. Details will be described later,
Excitation light components not absorbed by the EDF 16 enter the EDF 26 via the optical circulator 18, the fiber grating 22, and the optical isolator 24. Also, E
The DF 16 generates C-band ASE light by the excitation light 20a from the LD 20. This C-band ASE light also enters the EDF 26 via the optical circulator 18, the fiber grating 22, and the optical isolator 24. The pumping light 20a output from the LD 20 and the C-band ASE light generated in the EDF 16 assist the L-band amplification in the EDF 26, and help to improve the L-band amplification gain and / or reduce the noise figure. In order to introduce the C-band ASE light generated by the EDF 14 into the EDF 26, the C-band reflectivity of the fiber grating 22 is set to less than 100%. In the experiment, good results were obtained even when the C-band reflectivity of the fiber grating 22 was set to about 90%.

The output wavelength of the LD 20 is 980 nm when the optical fiber serving as the base of the EDF 16 is a quartz fiber.
Or, it is 1480 nm in the case of a fluoride optical fiber. Similarly, the output wavelength of the LD 38 is 980 nm or 1480 nm when the optical fiber serving as the base of the EDF 26 is a quartz fiber, and 1480 nm when the optical fiber is a fluoride optical fiber.

[0051] describing the amplifying operation of the C-band multiplexed optical signal S _C in the present embodiment. The signal light _{S C} is WD from the input terminal 10
The light enters the M optical coupler 12. WDM optical coupler 12
Transmits the pumping light 20a output from the LD 20 to the input terminal 1
Multiplexes the signal light _{S C} from 0, and supplies the multiplexed light to the EDF16 through the optical isolator 14. EDF16
It is excited by the excitation light 20a from the LD 20, for optically amplifying signal light _{S C.} At that time, the EDF 16 generates C-band ASE light. The output power of the LD 20 is ED
Because it is largely set enough to not be absorbed by the F16, EDF 16 is supplied with the signal light S _C that optical amplification, and C-band ASE light, the excitation light 20a which has not been absorbed into the port A of the optical circulator 18 I do.

The optical circulator 18 outputs the light input from the EDF 16 to the port A from the port B.
The signal light S _C , the C-band ASE light, and the pump light 20 a output from the DF 16 enter the fiber grating 22. C-band reflectivity less than 100% of the fiber grating 22, preferably so is about 90%, the fiber grating 22, the port of the signal light _{S C} and C-band optical circulator 18 most of the ASE light B
Reflected towards, and supplies the remaining excitation light 20a of the signal light _{S C} and C-band ASE light to the optical isolator 24.

[0053] the signal light _{S C} and C-band ASE light reflected by the fiber grating 22 is transferred from the port B of the optical circulator 18 to port C, the optical fiber 3
6, the light enters the port A of the optical circulator 34. Here the C-band ASE light at becomes noise light for the signal light _{S C.} Optical circulator 34 supplies the signal light _{S C} and C-band ASE light is inputted to the port A from the port B to the fiber grating 32. Since the fiber grating 32 is provided with 100% reflectivity for C-band, the signal light _{S C} and C-band ASE light,
Returning to port B of optical circulator 34 again, port C
To the output terminal 39.

In this embodiment, the L-band signal light S_LIncrease
The width operation will be described. Signal light S_LIs WD from input terminal 10
M optical coupler 12, optical isolator 14, and EDF 16
To the port A of the optical circulator 18
You. The excitation light 20a from the LD 20 is spread over the entire area of the EDF 16.
If it is strong enough to exist, EDF16
Is an L-band signal light S, as described later._LSlightly amplified
I do. The EDF 16 has an L-band signal light S_LDo not attenuate
However, as described later, the L-band signal light S _LTo
Slight amplification reduces noise figure of L-band amplification
it can.

As described above, the optical circulator 18
Since the output light to be input from the EDF16 to port A from the port B, the signal light _{S L} is an optical circulator 18
From the port B of the fiber grating 22. The fiber grating 22 is designed to reflect the C band, but slightly reflects part of the L band. FIG. 2 shows the reflection characteristics of the fiber grating 22. When the C band and the L band are sufficiently separated,
For example, when the distance is about 10 nm, the bottom portion of the reflection characteristics of the fiber grating 22 has no effect. However, the distance between the C band and the L band is very small, about 2 nm, and the C band and the L band are small. If the bands are substantially continuous, the fiber grating 22
Skirt of the reflection characteristic of also affects the L-band signal light S _L. The optical fiber 36 is provided to reduce this effect.

Therefore, the port B of the optical circulator 18
L-band signal light _{S L} output from the passes through the fiber gratings 22 and optical isolator 24, EDF
26. As described above, a part of the amplified C-band optical signal _{S C} in EDF 16, the remaining C-band ASE light and a portion of the excitation light 20a generated by EDF 14, then transmitted through the fiber grating 22, the light The light enters the EDF 26 via the isolator 24. EDF26
, It, is excited by the C-band multiplexed optical signal S _C and C-band excitation light 38a under ASE light auxiliary, efficiently L
The band signal light _SL is amplified. The wavelength of the excitation light 20a and E
Depending on the composition of the DF 26, the excitation light 20a is also
6 can be an excitation light for the L-band signal light SL. L
As described later, the power of the C-band light required by the EDF 26 as the auxiliary light for band amplification may be -30 dBm or more, and at most -1 dBm. Therefore, the C-band reflectivity of the fiber grating 22 may be about 90%. By inputting the weak C-band light to the EDF 26 as the auxiliary light, a wide band, high gain, and low noise can be achieved by L-band amplification of the EDF 26.

[0057] EDF26 generates the C-band ASE light in the course of amplifying the L-band signal light _{S L.} The C-band ASE light generated in the EDF 26 and the L band amplified in the EDF 26
Band signal light S _L is a WDM optical coupler 28 and optical isolator 30 transmits through lossless or low loss, and is incident on the fiber grating 32. Since the fiber grating 32 reflects C-band light 100% passes through only the amplified L-band signal light _{S L,} to reflect the C-band ASE light. The reflected C-band ASE light is absorbed by the optical isolator 30 or emitted to the outside.

[0058] The amplified L-band signal light S _L is transmitted through the fiber grating 32 is incident on port B of the optical isolator 34, is output from the port C to the output terminal 39.

[0059] L-band signal light _{S L} that is slightly reflected by the fiber grating 22, the port B of the optical circulator 18, C, optical fiber 36, and port A of the optical circulator 34, the fiber grating 3 through the B
2 is incident. Although the reflectivity is different, the reflection characteristics of the fiber grating 32 extend in the L band, so that the L output from the port B of the optical circulator 34
Some of the band signal light _{S L} is the fiber grating 32
At the port B of the optical circulator 34, and is supplied from the port C to the output terminal 39.

[0060] Thus, to the outside from the output terminal 39, and the C-band multiplexed optical signal _{S C} that is optically amplified by the EDF 14,
L-band signal light _{S L} which is optically amplified by EDF26 is output.

Most of the L-band signal components close to the C-band pass through the EDF 26, but a very small portion passes through the optical fiber 36. This is because the cutoff performance of the reflection characteristics of the fiber grating 22 is poor, as shown in FIG. In the present embodiment, the port C of the optical circulator 18 is used to compensate for the optical path difference between the two optical paths.
L of both paths between the optical circulator 34 and port A
An optical fiber 36 for matching the optical path length of the band light is connected. Therefore, the C band and the L band are so large that the reflection characteristics of the fiber gratings 22 and 32 do not matter.
If the bands are far apart, or if a reflector having a sufficiently steep cutoff performance can be used instead of the fiber gratings 22, 32, the port C of the optical circulator 18 should be connected to the port A of the optical circulator. You can connect directly. In this case, one four-port optical circulator can be used instead of two three-port optical circulators 18 and 34.

The inventor of the present application has found that even with a short EDF suitable for C-band amplification, when sufficiently strong excitation light is input, L-band optical amplification can occur. , C band light as seed light or auxiliary light
It has been found that entering the DF is beneficial. The contents will be described briefly.

As shown in FIG. 3, the mechanism of L-band amplification is to reabsorb 1.5 μm band ASE light generated by 1.48 μm or 0.98 μm excitation,
It is considered that the population inversion corresponding to the L band is formed. The L-band amplification EDF is long (for example, 150 m), and light of a wavelength having a large absorption coefficient is rapidly attenuated after incidence. From the absorption wavelength characteristic of the long EDF, it is considered that the reabsorption of the 1.5 μm band ASE light causes the amplification of the L band. But with this mechanism,
Since a sufficient population inversion is not formed for the L band,
Noise increases.

On the other hand, as a result of observation with the excitation light intensity sufficiently large, it was confirmed that L band amplification was caused by direct excitation of the excitation light as in the case of C band amplification, as shown in FIG. did. With this mechanism, there is a possibility that L-band amplification can be achieved with noise as inferior to C-band amplifiers. Specifically, the difference between the gain and the noise figure was examined for a 15 m long EDF in which the pump laser light is sufficiently strong over the entire length of the EDF and a 150 m long EDF in which the pump laser light is attenuated and disappears on the way. FIG. 5 shows the measurement results. The vertical axis indicates gain and noise figure, and the horizontal axis indicates wavelength. EDF used was 1000p of Er in quartz fiber.
pm-doped. For the short (15 m) EDF, the core diameter was designed to be small and the ratio of the MFD to the core diameter was designed to be large so that the pump laser light propagates as long as possible. When the core diameter is small, the number of absorbers in the core is also small, and the absorption of the excitation light is likely to be saturated. As a result, the pump light can propagate far.

In the experimental example, 15 mE having a core diameter of 2.4 μm was used.
DF was used. M at 1480 nm of the EDF
The FD / core diameter ratio is 1.93. Set the excitation light wavelength to 14
80 nm. For a 15 m long EDF, the incident excitation power was 80 mW, and the exit excitation power was 48 mW. Fifteen
For a 0 m long EDF, the incident excitation power was 190 mW and the exit excitation power was 0 mW. That is, in the 15 m length EDF, the total length is strongly excited at 48 mW or more, while in the 150 m length EDF, the 1480 nm light is attenuated and disappears on the way.

As can be seen from FIG. 5, with the 15 m long EDF, low noise of less than 5 dB was obtained over a wide wavelength range from 1540 nm to 1620 nm despite the excitation at 1480 nm. In addition, in comparison with the C band, the gain is positive at a wavelength of 1625 nm or less although it is small. In other words, even when the excitation is performed at 1480 nm, if the excitation is strongly performed over the entire length of the EDF, low noise of 5 dB or less can be obtained.
The L band positive gain is obtained. On the other hand, 150
In the m-length EDF, the gain is large, but the noise figure is also 6 dB.
It becomes big as above. This seems to be because the mechanism of L-band amplification shown in FIG. 3 functions in an EDF that is long enough to extinguish the excitation light.

On the basis of such measurement results, in the present embodiment, the output power of the LD 20 was increased over the entire area of the EDF 16 to an extent that excitation light could exist. As a result, the EDF 16 has a positive gain with respect to the L-band signal light and has low noise (noise of 5 dB or less). Excitation light components not absorbed by the EDF 16 pass through the optical isolator 18, the fiber grating 22, and the optical isolator 24 and enter the EDF 26, where they are absorbed and used as excitation light for amplifying L-band signal light. Is done.

Further, -30 dBm at which the EDF 14 is generated
The introduction of the C-band ASE light into the EDF 26 is as follows.
This is effective for increasing the L-band amplification gain and reducing noise in the EDF 26. The gain coefficient of the L band is lower than that of the C band. Low gain contributes to high noise. If the main mechanism of the L-band amplification is based on the reabsorption of the 1.55 μm band ASE light as shown in FIG. 3, the 1.55 μm band ASE light may be emitted more efficiently. Doing so increases the pumping efficiency, which increases gain and reduces noise. In order to emit 1.55 μm band light more efficiently, 1.55 μm band light may be introduced from the outside as a stimulus for light emission.

The dependence of gain on the wavelength of light introduced from the outside was examined. FIG. 6 shows the measurement results. The horizontal axis indicates wavelength, and the vertical axis indicates gain. An erbium-doped fluoride fiber having a length of 25 m was used, and the excitation wavelength was set to 1480.
nm. The wavelengths of the C-band light incident along with the excitation light are 1530 nm, 1540 nm, 1545 nm, 1
The gain characteristics in each case of 550 nm and 1560 nm and the gain characteristics of only the pump light were measured for comparison. As can be easily understood from FIG. 6, at any wavelength, the gain is increased by externally inputting the C-band auxiliary light. In particular, a wavelength of 1530 nm to 1550 nm
Is effective for increasing the gain.

The difference in the basic composition of the EDF, specifically, the difference between the erbium-doped fluoride optical fiber (EDFF) and the erbium-doped quartz optical fiber (EDSF) was confirmed. FIG. 7 shows the noise figure NF when the EDFF is used.
And gain. FIG. 8 shows the noise figure NF and the gain when EDSF is used. In each case, light having a wavelength of 1540 nm was incident on the EDF as auxiliary light. For comparison, the measurement result when no auxiliary light is incident is shown by a broken line.
7 and 8, the vertical axis represents the gain and the noise figure NF, and the horizontal axis represents the wavelength.

In FIG. 7, a characteristic curve 40 shows a gain characteristic when the C-band auxiliary light is incident, and a characteristic curve 42 shows a C-band auxiliary light.
The noise figure when the band auxiliary light is incident is shown. Also,
A characteristic curve 44 indicates a gain characteristic when the C-band auxiliary light is not incident, and a characteristic curve 46 indicates a noise figure when the C-band auxiliary light is not incident. From FIG. 7, when the C-band auxiliary light is incident, the gain is greatly increased as compared with the case where the C-band auxiliary light is not incident, and the noise figure is 1560 to
Although it increases in the wavelength range of 1570 nm, it can be seen that there is almost no difference in the wavelength range of 1570 to 1600 nm, which is the L band.

In FIG. 8, a characteristic curve 50 indicates a gain characteristic when the C-band auxiliary light is incident, and a characteristic curve 52 indicates a C characteristic.
The noise figure when the band auxiliary light is incident is shown. Also,
A characteristic curve 54 shows a gain characteristic when the C-band auxiliary light is not incident, and a characteristic curve 56 shows a noise figure when the C-band auxiliary light is not incident. From FIG. 8, it can be seen that both the gain and the noise figure are improved when the C-band auxiliary light is incident, as compared with the case where the C-band auxiliary light is not incident.

When the EDFF and the EDSF are compared, the following can be said. That is, when EDSF is used, C
Even in the presence of the band auxiliary light, the noise figure increases in the short wavelength region of 1580 nm or less, so that even if the gain is improved, it is difficult to expand the amplification band to 1570 nm or less. On the other hand, when EDFF is used, 1570 nm to 1600
There is no significant increase in the noise figure in the nm range. Therefore, 1
If the increase in noise at 560 to 1570 nm can be suppressed, a broadband L-band amplifier of 1560 to 1600 nm may be realized. Therefore, the EDF 26 is preferably based on a fluoride optical fiber.

When EDFF is used, 1560 to 1
Examining the factors that increase the noise figure at 570 nm,
This is because the characteristics of the optical filter that extracts the target wavelength band are insufficient, and 1550 to 1570n deviated from the center wavelength.
This was due to the presence of light of insignificant intensity in m.

The power required as auxiliary light was measured. FIG. 9 shows the measurement results. The wavelength of the auxiliary light is 1545 nm
It is. Even when the auxiliary light power was as low as -30 dBm, the effect of increasing the gain was confirmed. When the auxiliary light power is -1 dBm or more, the gain increasing effect is saturated.
That is, the maximum effect can be obtained even with a low power of -1 dBm.

The EDF 26 is the C that is generated by the EDF 16.
C band AS by stimulated emission light by band ASE light
E light is generated efficiently. That is, the EDF 26 is L
It is excited by two wavelengths including the excitation light 38a output from the D38 and the C-band ASE light generated by the EDF 16. C-band ASE generated by two-wavelength excitation
By re-absorption of light, the EDF 26 optically amplifies the L-band signal light with high gain and low noise. When the fiber grating 22 can transmit the pump light 20a with no loss or low loss, the component of the pump light 20a not absorbed by the EDF 16 can be used for pumping the EDF 26. This is also
It contributes to improvement of excitation efficiency.

[0077] A portion consisting of optical circulator 18 and fiber grating 22 basically functions as a wavelength demultiplexer to separate the C-band multiplexed optical signal S _C and L-band multiplexed optical signal S _L. However, in this embodiment, in particular, by intentionally leak the part of the C-band component (C-band ASE light generated in the C-band multiplexed optical signal S _C and EDF 14) on the output side of the L-band signal light S _L This is different from the ordinary wavelength demultiplexer.

It is easy to change the reflectivity of the fiber grating 22 in a desired shape with respect to the wavelength. E
When the C-band gain of the DF 16 has wavelength dependence, the wavelength dependence of the reflectance of the fiber grating 22 is set so as to flatten the wavelength dependence, so that a plurality of signal lights in the C band can be used. Thus, it is possible to obtain an amplification characteristic having a constant gain.

Further, the fiber grating has a C pitch by adjusting the grating pitch in the optical axis direction.
It reflects only each of the plurality of wavelength division multiplexed signal lights in the band, and reduces the ASE light between adjacent signal lights by approximately 10
0% transmission can be achieved. Fiber grating 2
The use of such a fiber grating as 2, high amplified signal light S _C C-band S
It can be extracted at the N ratio. As mentioned above, EDF
In order to improve the L-band amplification performance in 26, only the C-band ASE light is sufficient, and the C-band signal light may not be provided.

[0080] A portion consisting of a fiber grating 32 and optical circulator 34 functions as a multiplexer for multiplexing the are C-band multiplexed optical signal S _C and the optical amplification is optically amplified L-band signal light S _L. Therefore, it is apparent that the fiber grating 32 and the optical circulator 34 can be replaced by an optical coupler having a simpler configuration or a WDM optical coupler.

Instead of the LDs 20 and 38, a configuration may be adopted in which the output light of a single LD or the light obtained by combining the output lights of a plurality of LDs is split into two and supplied to the WDM optical couplers 12 and 38, respectively. it can. Of course, the oscillation wavelength of each LD must be in accordance with the EDF to be excited.

In the embodiment shown in FIG. 1, the excitation light of the EDF 26 is introduced from the rear. Unlike the embodiment shown in FIG. 1, the EDF 26 may be excited from the front. In that case,
The WDM optical coupler for introducing the pumping light output from the LD 38 into the EDF 26 includes the EDF 26 and the optical isolator 24.
, The excitation light 20a in terms of excitation efficiency
Is preferably able to pass through the WDM optical coupler, but the wavelength of the pump light 20a and the transmission characteristics of the WDM optical coupler are limited. In the embodiment shown in FIG. 1, such a restriction does not occur.

The excitation light 20a incident on the EDF 26 is L
If the power is sufficient for band amplification, the WDM optical coupler 28 and LD 38 can be omitted. This configuration is L
This is effective when the number of signal lights included in the band is small.

In order to set the EDF 16 to a bidirectional excitation state in which the EDF 16 is also excited from behind, for example, a four-port optical circulator is arranged in place of the three-port optical circulator 18, and the third excitation is provided in the fourth port. You just need to input light.

The parts of the optical circulators 18 and 34 are
Can be replaced by a port optical circulator. In this case, the optical fiber 3 to compensate for the delay of the L-band signal light S _L
You can not place a 6, but the insertion loss of the L-band signal light S _L increases slightly, the insertion loss of the C-band multiplexed optical signal S _C becomes smaller. Overall, rather than an increase in L-band insertion loss,
Reduction of insertion loss of the C-band multiplexed optical signal S _C is large. Thus, the benefits of reducing the insertion loss of the C-band multiplexed optical signal light S _C is greater than a disadvantage for the L-band signal light.

FIG. 10 is a block diagram showing a schematic configuration of such a modified embodiment. The same components as those in FIG. 1 are denoted by the same reference numerals. 60 is the optical circulators 18 and 34
4 port A, B, C, and D optical circulators that replace the part. That is, the EDF 16 is connected to the port A of the optical circulator 60, the fiber grating 22 is connected to the port B, and the fiber grating 32 is connected to the port C. Port D of optical circulator 60
Is connected to an output terminal 64 via an optical isolator 62. The optical isolator 62 is arranged in a direction in which the light output from the port D of the optical circulator 60 passes through the output terminal 64.

In the configuration shown in FIG. 10, the optical circulator 60 supplies the light input from the EDF 16 to the port A to the fiber grating 22 from the port B. The C band component reflected by the fiber grating 22 is input to the port B of the optical circulator 60 again,
Are output to the fiber grating 32.
The fiber grating 32 includes an optical circulator 60.
C light from the port C is totally reflected. The C-band light reflected by the fiber grating 32 enters the port C of the optical circulator 60 again.

The fiber grating 22 includes, among the light from the port B of the optical circulator 60, a part of the L-band signal light SL, a part of the C-band signal light SC, a part of the C-band ASE light generated by the EDF 16, and the pump light 20a. Through. The transmitted light is transmitted through the optical isolator 24 to the EDF.
Input to 26. The EDF 26 amplifies the L-band signal light SL by the same operation as the embodiment shown in FIG. The L-band signal light SL amplified by the EDF 26 is transmitted to the WDM optical coupler 2.
8. Optical isolator 30 and fiber grating 3
2 and is input to the port C of the optical circulator 60.

[0089] Thus, the port C of the optical circulator 60, the amplified L-band signal light _{S L} is input in the C-band multiplexed optical signal _{S C} and EDF26 amplified by the EDF 14. The optical circulator 60 outputs the input light of the port C from the port D. Therefore, the amplified C-band signal light S
_C and L-band multiplexed optical signal _{S L} is supplied to the output terminal 64 through the optical isolator 62.

When the optical circulator 60 has a function of transferring the input light of the port D to the port A, the pump light of the EDF 16 may be input to the port D of the optical circulator 60. For this purpose, a WDM optical coupler for supplying excitation light to the port D of the optical circulator 60 is disposed between the port D of the optical circulator 60 and the optical isolator 62.

Although the embodiment using erbium as an additive has been described, it is needless to say that other rare earth metals may be added.

[0092]

As can be easily understood from the above description, according to the present invention, signal light of two bands can be efficiently amplified with a simple configuration. In addition, high efficiency, high gain, and low noise figure can be realized, and a wide amplification band can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a first embodiment of the present invention.

FIG. 2 is a reflection characteristic diagram of the fiber gratings 22 and 30.

FIG. 3 is a diagram illustrating the principle of L band amplification for a long EDF.

FIG. 4 is a diagram illustrating the principle of L-band amplification for high-power excitation.

FIG. 5 is a measurement result example of a difference between a gain characteristic and a noise figure of an EDF depending on a length.

FIG. 6 is a measurement example of a relationship between a wavelength of an auxiliary light and a gain characteristic.

FIG. 7 is a graph showing wavelength characteristics of gain and noise figure when an EDFF is used.

FIG. 8 shows wavelength characteristics of gain and noise figure when using EDSF.

FIG. 9 is a diagram illustrating a relationship between auxiliary light power and gain.

FIG. 10 is a schematic configuration block diagram of a second embodiment of the present invention.

[Explanation of symbols]

 10: Input terminal 12: WDM optical coupler 14: Optical isolator 16: Erbium-doped optical fiber (EDF) 18: Optical circulator 20: Laser diode 20a: Excitation light 22: Fiber grating 24: Optical isolator 26: Erbium-doped optical fiber (EDF) 28: WDM optical coupler 30: optical isolator 32: fiber grating 34: optical circulator 36: optical fiber 38: laser diode 38a: pump light 39: output terminal 40: gain characteristic when C-band auxiliary light is input 42: C Noise figure when band auxiliary light is input 44: Gain characteristic when C band auxiliary light is not input 46: Noise figure when C band auxiliary light is not input 50: Gain characteristic when C band auxiliary light is input 52 : C-band auxiliary light input Noise figure 54 in the case that: C gain characteristics when the band auxiliary light does not enter 56: noise figure when C-band auxiliary light does not enter 60: optical circulator 62: optical isolator 64: Output terminal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/18 (72) Inventor Tomomi Sudo 2-1-1-15 Ohara, Kamifukuoka-shi, Saitama K.K. Inside Didy Fiber Lab (72) Inventor Yukio Noda 2-1-1-15 Ohara, Kamifukuoka City, Saitama Prefecture Inside Kay Di Fiber Lab (72) Inventor Eiki Mimura 2-1-1 Ohara, Kamifukuoka City, Saitama Prefecture F-term in C.D. Fiber Lab. (Reference) 5F072 AB09 AK06 JJ02 KK07 KK30 PP07 RR01 YY17 5K002 BA00 BA05 BA13 BA21 CA13 DA02 FA01

Claims (20)

[Claims] 1. A signal light _{(S C)} a signal light including signal light of the second band _{(L) (S} L) of the first band (C) is inputted, the signal light of the first band (C) A first optical amplifying medium for amplifying (S _C ); and a first optical amplifying medium of the light output from the first optical amplifying medium.
A splitter (18, 2) that supplies most of the band light to a first optical path (36) and supplies the rest to a second optical path.
2) a second optical amplifying medium (26) on the second optical path for amplifying the signal light (S _L ) of the second band (L).
And light propagating through the first optical path, and signal light (S _L ) of the second band (L) amplified by the second optical amplification medium.
And a multiplexer (32, 34) for multiplexing the signals and supplying the multiplexed signals to an output terminal. 2. A first reflector (22) connected to an input end of the second optical path for reflecting light of the first band at a reflectance of less than 100%, the first reflector comprising: The light output from the first optical amplifying medium (16) is transmitted to the first reflector (22).
And the light from the first reflector (22) is transferred to the first
To the first optical circulator (1)
8) The optical amplifier according to claim 1, comprising: 3. The combiner is connected to an output end of the second optical path and outputs the light of the first band (C) substantially 100 times.
% Reflecting second reflector and the light from the first optical path to the second reflector (32), and the light from the second reflector (32) to the output terminal. The optical amplifier according to claim 1 or 2, further comprising a second optical circulator (34) for transmitting. 4. A first pumping light supply device (12, 12) for supplying a first pumping light to the first optical amplification medium (16).
The optical amplifier according to claim 1, comprising: (20). 5. The first excitation light supply device (12, 2)
The optical amplifier according to claim 4, wherein 0) outputs the first pumping light with such an optical power that the first pumping light exists over the entire length of the first optical amplifying medium (16). 6. The optical amplifier according to claim 4, wherein said first optical amplification medium (16) has a positive gain for said second band (L) under said first pump light. . 7. A second pumping light supply device (28, 2) for supplying a second pumping light to the second optical amplifying medium (26).
38. The optical amplifier according to claim 1, comprising: 8. The optical amplifier according to claim 1, wherein said first optical path comprises an optical transmission medium (36) for compensating for a delay of said second optical path in said second band (L). 9. The optical amplifier according to claim 1, wherein the first band is a C band and the second band is an L band. 10. The optical amplifier according to claim 9, wherein said second optical amplification medium comprises a fluoride fiber. 11. The signal light is input including the signal light of the first band signal light (C) _{(S C)} and the second band _{(L) (S} L), the signal light of the first band (C) and (S _C) a first optical amplifying medium for amplifying (16), comprising a first, second, third and fourth ports, and outputs the input light of the first port from the second port, the second An optical circulator that outputs input light of a port from a third port and outputs input light of a third port from a fourth port,
An optical circulator for inputting light from the first optical amplifying medium (16) to a port; and a second amplifying the signal light (S _L ) of the second band (L).
An optical transmission line having an optical amplifying medium (26), and a first port that is disposed between the optical transmission line and a second port of the optical circulator and reflects light of the second band with a reflectance of less than 100%. A reflector (22), disposed between the optical transmission line and the third port of the optical circulator, for substantially reducing 100% of the signal light of the second band.
An optical amplifier comprising: a second reflector (32) that reflects light at a reflectance of (2). 12. The optical circulator according to claim 1, wherein said first and second optical circulators are
And second and third optical circulators that output input light of the first port from the second port and output input light of the second port from the third port. Is the first port of the optical circulator, the second port of the second optical circulator is the second port of the optical circulator, and the third port of the second optical circulator is the third port of the third optical circulator.
A first port of the optical circulator, wherein a second port of the third optical circulator is a third port of the optical circulator, and a third port of the third optical circulator is a fourth port of the optical circulator. Item 1
2. The optical amplifier according to 1. 13. An optical transmission line disposed between a third port of the second optical circulator and a first port of the third optical circulator to compensate for a delay of the optical transmission line in the second band (L). An optical amplifier according to claim 12, comprising an optical transmission medium (36). 14. The first optical amplifying medium (16).
Excitation light supply device (1) for supplying the first excitation light to
The optical amplifier according to claim 11, comprising (2, 20). 15. The first excitation light supply device (12, 2)
15. The optical amplifier according to claim 14, wherein 0) outputs the first pumping light with such an optical power that the first pumping light exists over the entire length of the first optical amplifying medium (16). 16. The optical amplifier according to claim 14, wherein the first optical amplification medium (16) has a positive gain with respect to the second band (L) under the first pump light. . 17. The first reflector (22), wherein the first reflector (22) is
15. The optical amplifier according to claim 14, wherein the optical amplifier transmits the excitation light. 18. The second optical amplifying medium (26).
Excitation light supply device (2) for supplying the second excitation light to the
The optical amplifier according to claim 11, comprising (8, 38). 19. The optical amplifier according to claim 11, wherein said first band is a C band, and said second band is an L band. 20. The optical amplifier according to claim 19, wherein said second optical amplifying medium comprises a fluoride fiber.