JPH10107351A - Er-doped optical fiber amplifier for bidirectional transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture an Er-doped optical fiber amplifier for bidirectional transmission, having flat-gain wavelength characteristics extending over a wide wavelength region. SOLUTION: An Er-doped optical fiber amplifier 109 is connected between an optical circulator 101, connected to a phototransmitter 103 and a photoreceptor 105 and another optical circulator 113 connected to another photoreceptor 116 and another phototransmitter 118 through the intermediary of a WDM filter 115, so as to emit the excited beams from excited beam sources 108 and 112. On the other hand, the optical circulator 113 is connected to another Er-doped optical fiber 114, so as to suppress the specific wavelength band of the signal beams from the Er-doped optical fiber 109. Through these procedures, the amplifier gain of the signal beams in each wavelength can be made almost equal to the specifications of the phototransmitters and the photoreceptors, thereby making feasible the enhancement in mass productivity and cost reduction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Er-doped optical fiber amplifier used in a bidirectional transmission system for transmitting signal lights having different wavelengths in both directions.

[0002]

2. Description of the Related Art In recent years, optical fiber amplifiers in which a rare earth element such as Er (erbium), Pr (praseodymium), or Nd (neodymium) is added to the core of an optical fiber have reached a practical level. In particular, since an optical fiber amplifier doped with Er has a high gain and a high saturation output in a 1.55 μm band, application to various systems is considered. Among them, 1.53μm to 1.56μ
High-speed, large-capacity, long-distance transmission and optical CATV (Cable Televis) by wavelength multiplex transmission using several or more wavelengths of signal light in m wavelength band
Attention has been focused on application to ion) systems. In such a system, it is desired to be able to transmit data in one transmission path in both directions of up and down directions. As a bidirectional transmission system using an optical fiber amplifier, a system using an optical circulator as shown in FIGS. 7 and 8 has been proposed.

First, FIG. 7 will be described. The signal light A incident from the input / output port 701 is combined with the pump light from the pump light source 703 by the optical multiplexer 702. The output light of the optical multiplexer 702 propagates through the rare-earth-doped optical fiber 704, enters the port 705A of the optical circulator 705, passes through the inside, exits from the port 705B, and enters the optical filter 706. In the optical filter 706, only the excitation light is cut, and only the amplified signal light is transmitted to the optical fiber 7.
07 to the input / output port 708 and the signal light A
Is output as

On the other hand, when the signal light B is input to the input / output port 708, it enters the port 705 B of the optical circulator 705 through the optical filter 706, exits from the port 705 C of the optical circulator 705, and
At a reflection point 710. This reflected light exits to the port 705D through the optical fiber 709 and the optical circulator 705, and further enters the rare-earth-doped optical fiber 711. The signal light that has passed through the rare earth-doped optical fiber 711 travels straight through the optical multiplexer 713 and reaches the reflection point 714.
The light is reflected at the reflection point 714 and returned to the same optical path. At this time, the excitation light is generated by the excitation light source 712, and when the excitation light is reflected by the optical multiplexer 713, the excitation light is multiplexed with the signal light from the reflection point 714 and amplified in the process of passing through the rare-earth-doped optical fiber 711. Is done. This amplified light enters the port 705D of the optical circulator 705 and exits to the port 705A via the optical circulator 705. Further, the signal light is amplified when passing through the rare-earth-doped optical fiber 704, and the amplified signal light passes through the optical multiplexer 702 and is output from the input / output port 701 as the signal light B.

Next, the configuration of FIG. 8 will be described. FIG.
Has a configuration using two optical circulators and three rare earth-doped optical fibers, thereby improving the gain. In the figure, the configuration on the right side of the rare-earth-doped optical fiber 704 is the same as that in FIG. An optical circulator 716 is inserted between the optical multiplexer 702 and the rare earth-doped optical fiber 704. The optical multiplexer 702 is connected to a port 716B of an optical circulator 716 via an optical fiber 715, and a rare earth-doped optical fiber 704 is connected to a port 716C. The port 716C of the optical circulator 716 is connected to a reflection point 718 via an optical fiber 717, and the port 716D is connected to a rare earth-doped optical fiber 719. An optical multiplexer 720 is disposed at the other end of the rare earth-doped optical fiber 719, a reflection point 721 is disposed on a straight optical path, and an excitation light source 722 is disposed on a reflected optical path of the optical multiplexer 720. ing.

In this configuration, when the signal light A enters the optical circulator 716, the signal light A and the pumping light of the pumping light source 703 are multiplexed by the optical multiplexer 702, and the light enters the port 716B of the optical circulator 716. I do. Port 716B
Is emitted from the port 716A of the optical circulator 716, is amplified when passing through the rare-earth-doped optical fiber 704, and the amplified signal light is
05, sequentially pass through the optical filter 706, and
08 is output as signal light A.

The signal light B input from the input / output port 708 is amplified by the rare-earth-doped optical fibers 711 and 704 as described with reference to FIG. The light enters the circulator 716. The signal light B incident on the optical circulator 716 is optically amplified by the rare-earth-doped optical fiber 719, passes through the port 716 B, and is emitted to the optical fiber 715. The signal light B emitted to the optical fiber 715 passes through the optical multiplexer 702 and is output to the input / output port 701.

[0008]

However, according to the conventional rare earth-doped optical fiber amplifier for bidirectional transmission, 1.53
The signal light of the wavelength band of μm to 1.56 μm is 1 nm or 2 nm.
When several tens of waves are used at an interval of nm and the signal light A and B are to be transmitted bidirectionally, the gain in the above-mentioned wavelength band is not flat with the current configuration of the Er-doped optical fiber amplifier. The gain near the .53 μm band is 1.545 μm to 1.45 μm.
Since the gain over 5 μm is higher than the gain over 56 μm, there is a problem that the optical S / N characteristics (signal light-to-noise ratio characteristics) of the received signal light and the crosstalk characteristics between the respective signals are deteriorated.

Also, the deterioration of the optical S / N characteristics and the crosstalk characteristics reduces the number of signal light channels that can be transmitted from the A side and the B side, and reduces the amount of information that can be transmitted in both directions. There is a problem that restrictions occur.
Accordingly, it is an object of the present invention to provide an Er-doped optical fiber amplifier for bidirectional transmission having a flat gain wavelength characteristic over a wide wavelength range.

[0010]

In order to achieve the above object, the present invention provides first and second optical transmitters for outputting wavelength-multiplexed first and second signal lights, and at least three optical transmitters. A first optical receiver that has two ports, of which two ports receive the first and second signal lights and the other port receives the second signal light. One optical circulator, a second optical circulator having at least four ports, and an Er-doped optical fiber connected between the first optical circulator and the second optical circulator for performing bidirectional amplification A pumping means for supplying pumping light of a predetermined wavelength to the Er-doped optical fiber in one direction or bidirectional direction, and a signal light from the Er-doped optical fiber connected between two ports of the second optical circulator. A gain adjusting means for attenuating or reflecting a specific wavelength band in the second optical circulator, a second optical transmitter connected to another port of the second optical circulator, and a second optical receiver for receiving the first signal light. And a WDM filter to which the optical receiver is connected.

According to this configuration, the specific wavelength band of the amplified signal light is made higher than other wavelength bands by passing the signal light through an optical component having a characteristic of attenuating or reflecting a specific wavelength band of the signal light. Even when the gain is large, the gain deviation is reduced, and the amplification gains of the signal lights of the respective wavelengths can be made substantially equal. As a result, deterioration of the optical S / N characteristics (optical signal / noise characteristics) and optical crosstalk characteristics (optical crosstalk characteristics between signal lights of each wavelength) can be extremely reduced. Further, since the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, the specifications of the optical transmitter and the optical receiver can be made substantially the same, and the mass productivity can be improved and the cost can be reduced.

The gain adjusting means can use an unexcited Er-doped optical fiber, an interference film optical filter, or an optical fiber grating. According to this configuration, each has a characteristic of attenuating or reflecting a specific wavelength band of the signal light, and the amplification gain of the signal light of each wavelength can be made substantially equal. The first optical circulator includes a second WDM filter to which the first optical transmitter and the first optical receiver are connected, and a specific wavelength of signal light from the second optical transmitter. A configuration in which gain adjustment means having a characteristic of attenuating or reflecting the band can be connected.

According to this configuration, the amplification gain can be made substantially equal at each wavelength of the signal light received on both the upstream side and the downstream side. The above object has first and second optical transmitters for outputting wavelength-multiplexed first and second signal lights, and at least three ports, of which two ports are the first and second optical transmitters. Second
A first optical circulator connected to a first optical receiver for receiving the second signal light at another port while receiving the second signal light, and a second optical circulator including at least four ports.
An optical circulator, a first Er-doped optical fiber that is connected between the first optical circulator and the second optical circulator and performs bidirectional amplification, and a first optical circulator and a second optical circulator. A gain adjusting means connected to the first Er-doped optical fiber between the first and second optical circulators for attenuating or reflecting a specific wavelength band in the signal light; and a gain adjusting means connected to the gain adjusting means between the first and second optical circulators. A second Er-doped optical fiber for performing bidirectional amplification, a pumping means for supplying a pump light of a predetermined wavelength to the first and second Er-doped optical fibers in one direction or in two directions,
The present invention is also achieved by a configuration including a WDM filter connected to the second optical transmitter and a second optical receiver that receives the first signal light.

According to this configuration, an optical component having a characteristic of attenuating or reflecting a specific wavelength band in the signal light is disposed between the two Er-doped optical fibers for amplification, so that the upstream side and the downstream side are provided. Amplification can be performed on any of the above signal lights, and the amplification gains of the signal lights of the respective wavelengths can be made substantially equal. Therefore, the optical S / N characteristic (optical signal /
Noise characteristics) and optical crosstalk characteristics (optical crosstalk characteristics between signal lights of respective wavelengths) can be extremely reduced. Further, since the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, the specifications of the optical transmitter and the optical receiver can be made substantially the same, and the mass productivity can be improved and the cost can be reduced.

As the gain adjusting means, a non-excitation Er-doped optical fiber, an interference film optical filter, or an optical fiber grating can be used. According to this configuration, each has a characteristic of attenuating or reflecting a specific wavelength band of the signal light, and the amplification gain of the signal light of each wavelength can be made substantially equal.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing a first embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention, which uses a signal light wavelength from a wavelength range of 1.52 μm to 1.57 μm. Are suitable.

The three-terminal optical circulator 101 (first optical circulator) has three ports a, b, and c, and the port a has an optical transmitter 10 for transmitting the signal light 102.
3 (first optical transmitter), and an optical receiver 105 (first optical receiver) for receiving the signal light 104 is connected to the port c. Further, a port b of the optical circulator 101 is provided with WDM (Wavelength Division Multiplexing:
(Wavelength division multiplexing) coupler 106 is connected to the WDM coupler 106.
(WDM coupler 106 and excitation light source 10
8 constitutes the excitation means). In addition, the WDM coupler is a filter having a characteristic of passing the signal light as it is and demultiplexing the pump light.

Further, the WDM coupler 106 has an Er-doped optical fiber 109 (the length of which is selected in the range of from several tens of meters to thirty and several meters.
and the Er-doped optical fiber 109 is connected to the WDM coupler 1.
10 is connected, and the pumping light 111 from the pumping light source 112 is injected (the WDM coupler 110 and the pumping light source 112 constitute a pumping unit). The port a of the optical circulator 113 is connected to the WDM coupler 110. W
Er between port b and port c of the DM coupler 110
The additional optical fiber 114 (optical component) is connected. Further, a WDM filter 115 is connected to the port d, and the WDM filter 115 is connected to an optical receiver 116 (second optical receiver) for receiving the signal light 117 and an optical transmitter 118 (second optical receiver) for sending out the signal light 119. 2 optical transmitters) are connected.

In the above configuration, the optical transmitter 103 (T
X ₁ ) uses several to several tens of channels of signal light containing at least one signal light having a wavelength on the short wavelength side near 1.53 μm, and transmits information on each of these signal lights. These signal lights are finally transmitted to the optical receiver 116.
(RX ₁ ). On the other hand, from the optical transmitter 118, signal light that does not include signal light having a shorter wavelength near 1.53 μm, for example, signal light using a longer wavelength of 1.547 μm to 1.57 μm is transmitted by several channels. And several tens of channels, and transmits these signals with their respective information. These signal lights are finally transmitted to the optical receiver 105.
(RX ₂ ). When bidirectional transmission is performed using the above-mentioned wavelength band, the signal lights of the respective wavelengths are amplified with substantially the same amplification degree, and the optical receivers 105 and 116 are in a state where the signal light power levels of the respective wavelengths are substantially equal. Is entered. Next, details of the operation in FIG. 1 will be described.

From the optical transmitter 103, wavelength-multiplexed signal light 102 (including signal light of 11 channels at 2 nm intervals in a wavelength range of 1.526 μm to 1.546 μm,
Wavelength multiplexed with information on each signal light) is transmitted. The signal light 102 is input to the port a of the optical circulator 101, passes through the inside, is emitted from the port b, and further enters the WDM coupler 106. The WDM coupler 106 receives the pump light 107 (wavelength 0.
(98 μm band or 1.48 μm band) and the signal light 102 are combined, and the combined signal light 120 and pump light 107 are Er.
The light enters the additive optical fiber 109. Excitation light 107 is Er
The pumping light is absorbed by the Er ions by propagating in the doped optical fiber 109, and forms an inversion distribution state, thereby amplifying the signal light 120. Also, the pump light 111 (wavelength of 0.98 μm band or 1.48 μm band, the optical power is several tens mW to one hundred and several tens of meters) generated from the pump light source 112 from the other end (output end) side of the Er-doped optical fiber 109. mW) is incident on the Er-doped optical fiber 109, and amplifies the signal light 120. However, the pumping light 111 propagates in a direction opposite to the propagation direction of the pumping light 107. Most of the pump light 111 and 107 is absorbed in the Er-doped optical fiber 109. The slight excitation light that has not been absorbed is the WDM coupler 106,
At 110, it is split and removed.

The signal light 120 is an Er-doped optical fiber 109.
Can have a gain of 25 dB or more. This gain depends on the optical power of the signal light 102 and the pump light 1
07, the amount of Er added to the Er-doped optical fiber 109, and the fiber length. That is, the signal light 102
The smaller the optical power of the signal light, the higher the gain, and the signal light 1
As the optical power of 02 increases, the gain tends to be saturated. Also, the gain increases as the optical power of the pump light 107, 111 increases.

If the amount of Er added to the Er-doped optical fiber 109 is 50 ppm or more, almost sufficient gain can be obtained, and the gain does not increase so much as the amount of Er added increases. The amount of Er added is 100 as described above.
A range from ppm to 1000 ppm is preferred. In addition, Er
1,000 ppm of Al in the doped optical fiber 109
Preferably, they are co-added in the range of ~ 60,000 ppm. Further, the noise figure can be reduced by making the length of the Er-doped optical fiber 109 shorter than the length at which the maximum gain is obtained (usually selected within a range of 30 to 50 m). More desirably, the fiber length is set so as to realize the maximum saturation output (selected within the range of 10 to 30 m). The signal light 121 amplified under the above conditions is in a state where the signal light near 1.53 μm is amplified with a gain 1 to 5 dB higher than the signal light of other wavelengths.

The signal light 121 enters the port a of the second optical circulator 113, passes through the inside of the optical circulator 113, is output from the port b, and is transmitted as the signal light 122 to the Er-doped optical fiber 114. Is done. Since no excitation light is applied to the Er-doped optical fiber 114, the signal light 122 is
Is transmitted as signal light 123 accompanied by a loss depending on the loss wavelength characteristic of the Er-doped optical fiber 114, and is input to the port c of the optical circulator 113.

The loss wavelength characteristic of the Er-doped optical fiber 114 is as shown in FIG. 3, for example, when an Er-doped multi-core optical fiber having a structure as shown in FIG. 2 is used. Er
As shown in FIG. 2, the structure of the doped multi-core optical fiber 200 is such that a primary clad layer 202 is covered and a glass rod 203 having a plurality of cores 201a to 201g co-doped with Er and Al is assembled. The periphery of 203 is thickly covered with a secondary clad 204. By using the Er-doped multi-core optical fiber 200 having such a structure, it is possible to achieve high gain and flatten the wavelength characteristic of gain.

As shown in FIG. 3, there is a loss peak near the wavelength of 1.53 μm, and the loss gradually decreases in the short wavelength and long wavelength regions. This characteristic is E
The larger the amount of r added, the larger the loss peak value.
The greater the amount of Al added, the more gradual the loss on the long wavelength side is reduced. In addition, the loss increases as the fiber length of the Er-doped optical fiber 114 increases. When the maximum value of the gain deviation between the channels of the signal light 123 is about 5 dB, the Er-doped optical fiber 114
The fiber length of No. 4 was about 1 m, and the amount of Er added was about 400 p.
pm, Al addition amount is about 5,000 ppm to 10,000
The one in the range of 00 ppm is used. As a result, Er
The output deviation of the signal light having the wavelength of each channel of the signal light 123 that has passed through the addition optical fiber 114 can be suppressed as small as possible.

The signal light 123 enters the port c of the optical circulator 113, passes through the optical circulator 113 and is output from the port d.
Input to the filter 115. The WDM filter 115 is an optical filter having a wavelength selecting function. The WDM filter 115 separates the signal light 117 from the wavelength-multiplexed signal light 124, and
16 is input. The optical receiver 116 converts the signal light 117 into an electric signal, and reproduces an information signal of each channel.

On the other hand, the wavelength multiplexed signal light 119 (for example, 1.547 μm to 1.547 μm) is transmitted from the optical transmitter 118 (TX ₂ ).
In the wavelength range of 1.561 μm, eight channels of signal light are included at intervals of 2 nm, and wavelength multiplexing is performed by transmitting information to each signal light.) The signal light 119 is multiplexed by the WDM filter 115. , As signal light 125, to port d of optical circulator 113. The signal light 125 passes from port d to port a, and propagates to the WDM coupler 110. In the WDM coupler 110, the excitation light source 1
The pump light 111 from 12 and the signal light 125 are multiplexed, and the signal light 127 and the pump light 111 are input to the Er-doped optical fiber 109.

The Er-doped optical fiber 109 receives the pump light 11
The signal light 127 is amplified using 1107 so as to have a gain of 25 dB or more. Amplified and the WDM coupler 106
Signal light 12 that has passed through the optical circulator 101
In No. 7, since the wavelength band is in the wavelength range of 1.547 μm to 1.561 μm, the signal light of each channel is amplified with almost the same gain. That is, the signal is amplified with almost no gain deviation. Therefore, there is no need to compensate for the gain deviation through the Er-doped optical fiber 114 as shown in FIG.

The amplified signal light 127 enters the port b of the optical circulator 101, passes through the inside, is output from the port c, and is received as the signal light 104 by the optical receiver 105. The light is split into respective channels in the optical receiver 105, converted into an electric signal by an optical-electrical conversion circuit in the optical receiver 105, and each information signal is reproduced. As mentioned above,
According to the Er-doped optical fiber amplifier of the present invention shown in FIG. 1, it is possible to construct a bidirectional transmission system capable of amplifying the signal light transmitted from each of the optical transmitters 103 and 118 with substantially equal gain. it can. As a result, the specifications of the optical transmitter and the optical receiver can be made substantially the same, and the mass productivity can be improved and the cost can be reduced. Further, the wavelengths of the upstream and downstream signal lights are set to 1.52 μm or more.
Since it can be freely selected in the range of 1.57 μm, flexibility and expandability of the system can be achieved. Furthermore, since the amplification gains of the signal light on the upstream side and the signal light on the downstream side can be made substantially equal, the transmission distances on the upstream side and the downstream side can also be made substantially equal, and the system design becomes easy. In addition, since the excitation method is symmetrical on the upstream side and the downstream side, the noise figure characteristics on the upstream side and the downstream side can be made substantially equal. That is,
The quality of the received information can be approximately equal in both directions. This is extremely effective especially for image transmission and ultra-high-speed information transmission.

FIG. 4 is a circuit diagram showing a second embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention. As with the Er-doped optical fiber amplifier for bidirectional transmission of FIG. It is suitable to use the wavelength of the signal light from the wavelength range of 52 μm to 1.57 μm. In FIG. 4, the same reference numerals are used for the same components as those shown in FIG. 1, and thus redundant description will be omitted below.

Here, the Er-doped optical fiber 10 shown in FIG.
At the place where 9 is provided, a non-pumped (does not transmit pump light) Er-doped optical fiber 128 (having a length of about 2 m) is disposed with WDM couplers 136 and 137 connected to both sides. (WDM couplers 129, 136, 1
37 and the excitation light sources 108 and 112 form excitation means). The Er-doped optical fiber 128 has the same function as the Er-doped optical fiber 114 shown in FIG. 1, and has a configuration in which the Er-doped optical fiber 114 is moved from the place in FIG.

Further, an Er-doped optical fiber 109a (first Er-doped optical fiber) having the same function as the Er-doped optical fiber 109 is provided between the WDM coupler 136 and the optical circulator 101, and the WDM coupler 137 and the optical circulator are provided. An Er-doped optical fiber 109b (second Er-doped optical fiber) having the same function as the Er-doped optical fiber 109 is provided between the 113. Then, the WDM coupler 136 and the WDM coupler 137 are coupled by a WDM coupler 129, and the WDM coupler 129 is connected to the excitation light source 108,
The excitation light 111 and 107 can be supplied to the WDM couplers 136 and 137. The other configuration is as shown in FIG. The Er-doped optical fiber 109a and the Er-doped optical fiber 109b may be Er-doped multi-core fibers having a structure as shown in FIG. Er-doped optical fiber 109
a, the length of 109b is about 10 m, the addition amounts of Er and Al are 400 ppm and 17,000 ppm, the diameter of each core is about 1.9 μm, the distance between the cores is about 1.3 μm,
When the relative refractive index difference between the core and the clad is about 2.1% and the power of the pumping light 111 (wavelength 0.98 μm) from the pumping light source 112 is about 80 mW, the signal light 130 is transmitted through the Er-doped optical fiber 109a. 26 dB (wavelength 1.538 μm-
It is amplified with a gain of 1.546 μm) to 32 dB (wavelength 1.53 μm) and output as signal light 131.

In the above configuration, as in the case of FIG. 1, 1.526 μm to 1.546 μm are transmitted from the optical transmitter 103.
In the wavelength range of μm, signal light of 11 channels is included at intervals of 2 nm, and wavelength-division multiplexed signal light is transmitted with information carried on each signal light. The signal light 102 is input to the port a of the optical circulator 101, passes through the inside, exits from the port b, and enters the Er-doped optical fiber 109a. An excitation light source 112 is provided in the Er-doped optical fiber 109a.
Pump light 111 generated by the WDM couplers 129 and 136
, And propagates through the Er-doped optical fiber 109a from the opposite direction to the signal light 130. Thereby, the signal light 130 is amplified.

As described above, when the Er-doped multi-core fiber having the structure shown in FIG. 2 is used, a gain deviation of about 6 dB occurs between channels within the above wavelength. Therefore, in order to suppress this gain deviation, the light is transmitted through the Er-doped optical fiber 128. Thereby, the gain deviation is 2 dB
Suppressed below, 1.53 μm gain is 1.538 μm
Conversely, it can be lower than the gain of ４６1.546 μm. The signal light whose gain deviation has been suppressed is transmitted to the Er-doped optical fiber 109b, and the second amplification is performed. The excitation light 1 from the excitation light source 108 is applied to the Er-doped optical fiber 109b.
07 is supplied after being split by the WDM couplers 129 and 137, and amplification is performed using the pump light 107.

When 0.98 μm is used for the pumping light 107 and the power is set to about 90 mW, the signal light 102 is set to about 39 mW.
It can be amplified to a gain of dB to 40 dB. Thus, by sequentially performing the first amplification, the gain deviation suppression, and the second amplification, the gain deviation of each channel could be suppressed to about 1 dB. Er-doped optical fiber 10
The signal light 132 output from the optical circulator 113 enters the port a of the optical circulator 113, exits from the port b, and
It is sent to DM filter 115. WDM filter 115
The signal light of 1.526 μm to 1.546 μm is demultiplexed, extracted as signal light 117, and received by the optical receiver 116.

On the other hand, the signal light 119 transmitted from the optical transmitter 118 is 1.547 μm
In the wavelength range of 1.561 μm, eight channels of signal light are included at intervals of 2 nm, and information is added to each signal light and wavelength multiplexed. The signal light 119 is multiplexed by the WDM filter 115, enters the port c of the optical circulator 113 as the signal light 125, passes through the inside, and exits from the port a. The emitted light is the Er-doped optical fiber 1
09b as signal light 133, and is amplified while passing through the Er-doped optical fiber 109b. Since the amplified signal light 134 does not include 1.53 μm in the short wavelength band, a substantially flat gain-wavelength characteristic can be obtained.

Although the signal light 134 is sent to the Er-doped optical fiber 128 via the WDM coupler 137, the gain in the channel is slightly reduced. Er-doped optical fiber 1
The output light of No. 28 is sent to the Er-doped optical fiber 109a,
A predetermined amplification is performed. The signal light 135 resulting from the amplification is input to the port b of the optical circulator 101, passes through the inside, and exits from the port c. The signal light 104 emitted from the port c is received by the optical receiver 105.

As described above, the Er-doped optical fiber amplifier for bidirectional transmission shown in FIG. 4 has the first amplification medium (E) between two optical circulators (optical circulators 101 and 113).
An r-doped optical fiber 109a), a loss medium (Er-doped optical fiber 128), and a second amplifying medium (Er-doped optical fiber 109b) are provided, and an optical receiver and an optical transmitter having different wavelengths are connected to two optical circulators. When the signal light from the optical transmitters 103 and 118 passes through the first amplifying medium → the loss medium → the second amplifying medium (or the reverse path), the signal light from the optical transmitters 103 and 118 is transmitted. The deviation of the amplification gain is suppressed. As a result, it is possible to construct a bidirectional transmission system capable of amplifying the signal light transmitted from each of the optical transmitters with substantially equal gain.

FIG. 5 is a circuit diagram showing a third embodiment of the Er-doped optical fiber amplifier for bidirectional transmission according to the present invention. In FIG. 5, the same reference numerals are used for the same components as those shown in FIGS. 1 and 4, and thus, duplicate description will be omitted below. The Er-doped optical fiber amplifier for bidirectional transmission shown in FIG. 5 is characterized in that the arrangement of the pump light source and the pump system in the configuration of FIG. 4 are different. The three-terminal optical circulator 101 is composed of three of a, b, and c.
The optical transmitter 103 for transmitting the signal light 102 is connected to the port a, and the signal light 10 is connected to the port c.
4 is connected to the optical receiver 105. Furthermore,
WDM coupler 1 is connected to port b of optical circulator 101.
06 is connected to this WDM coupler 106.
07 is connected. In addition, the WDM coupler is a filter having a characteristic of passing the signal light as it is and demultiplexing the pump light.

The WDM coupler 106 includes an Er-doped optical fiber 109a, a WDM coupler 136, an Er-doped optical fiber 128, a WDM coupler 137, and an Er-doped optical fiber 10.
9b, WDM coupler 110, and optical circulator 11
3 are sequentially connected in series. An excitation light source 108 is coupled to the WDM coupler 106, and an excitation light source 112 is coupled to the WDM coupler 110. Optical circulator 1
The WDM filter 138 is connected to the port b of thirteen,
The optical receiver 116 is connected to the WDM filter 138. A WDM filter 139 is connected to the port c of the optical circulator 113, and an optical transmitter 118 is connected to the WDM filter 139.

The signal light 102 from the optical transmitter 103 passes through the optical circulator 101 to become a signal light 130,
When passing through the r-doped optical fiber 109a, the excitation light 107
Amplification is performed based on The signal light 13 due to this amplification
1 indicates that the gain deviation is suppressed during transmission through the Er-doped optical fiber 128. Thereafter, the Er-doped optical fiber 109
b and is amplified by the Er-doped optical fiber 109b. The excitation light 107 from the excitation light source 108 is supplied to the WDM coupler 136
The light is supplied after being demultiplexed at 137, and amplification is performed using the pump light 107. The signal light resulting from the amplification is transmitted to the Er-doped optical fiber 109b, and the second amplification is performed using the pumping light 107 from the pumping light source. The signal light 132 is transmitted through the optical circulator 113 to the WDM
It is sent to the filter 138. The WDM filter 138 is an optical filter having a wavelength selection function. The WDM filter 138 separates the signal light 117 from the wavelength-multiplexed signal light 132, and
Send to 6. The optical receiver 116 converts the signal light 117 into an electric signal and reproduces an information signal of each channel.

The signal light 119 from the optical transmitter 118
, WDM filter 139 → port c of optical circulator 113 → port a → WDM coupler 110 → E
r-doped optical fiber 109b → WDM coupler 137 → Er
The signal light passes through the route of the addition optical fiber 128 → WDM coupler 136 → Er addition optical fiber 109a → WDM coupler 106 → optical circulator 101 → optical receiver 105. First amplification is performed using the pump light 111 in the Er-doped optical fiber 109b, and second amplification is performed using the pump light 111 in the Er-doped optical fiber 109a. Note that E
The pumping light 111 remaining without being absorbed in the r-doped optical fiber 109b is split by the WDM coupler 137,
The signals are multiplexed by the coupler 136.

The effect obtained in the configuration of FIG.
This is the same as the case of the configuration, and it is possible to construct a bidirectional transmission system capable of amplifying the signal light transmitted from each of the optical transmitters with almost equal gain. FIG. 6 is a circuit diagram showing a fourth embodiment of the Er-doped optical fiber amplifier for bidirectional transmission according to the present invention. In FIG. 6, the same reference numerals are used for the same components as those shown in FIG. 1, and thus redundant description will be omitted below.

The configuration shown in FIG. 6 performs bidirectional transmission using the wavelengths in the vicinity of the 1.55 μm band for the signal lights 102 and 119 transmitted from the optical transmitters 103 and 118.
The configuration on the right side of the M coupler 106 is the same. An optical circulator 140 having four terminals is used instead of the optical circulator 101 of FIG. 1, and an Er-doped optical fiber 109 is connected to the port b. Further, an Er-doped optical fiber 141 having the same function as the Er-doped optical fiber 114 is connected between the port c and the port d of the optical circulator 140. A channel-type WDM filter 142 is connected to the port a of the optical circulator 140, and the optical transmitter 103 and the optical receiver 105 are connected to the WDM filter 142. Further, instead of the WDM filter 115, a channel type WDM filter 142,
143 is used.

The wavelength of the signal light 102 is 1.528 μm.
m, 1.530 μm, 1.550 μm, 1.552 μ
m, 1.554 μm, 1.556 μm, 1.558 μm
And 1.560 μm can be used. The signal light 119 transmitted from the optical transmitter 118 includes 1.53
2 μm, 1.534 μm, 1.536 μm, 1.538
μm, 1.540 μm, 1.542 μm, 1.544 μ
m and 1.546 μm can be used. The WDM filters 142 and 143 are used to split or multiplex the signal light of 16 channels. Also, the Er-doped optical fiber 114 and the Er-doped optical fiber 141 are
It is used to reduce the wavelength near the 1.53 μm band included in the signal light on the receiving side and make the signal light of each channel have substantially the same gain.

The other configuration of FIG. 6 is the same as that described with reference to FIG. 1, and a description thereof will not be repeated. The effect obtained is the same as that of the configuration shown in FIG. The present invention is not limited to the configuration shown in each of the drawings.
For example, the Er-doped optical fibers 114, 128, and 141 of FIGS. 1, 4, 5, and 6 have characteristics that can flatten the optical power of each signal light in the wavelength band input to the optical receiver. Er-doped optical fibers 114, 12
Instead of 8, 141, an interference film optical filter, an optical fiber grating or the like can be used. That is, an optical filter that attenuates or reflects signal light having a wavelength of 1.53 μm, suppresses the reflected light, and transmits signal light having a wavelength other than 1.53 μm (band stop optical filter characteristic having a center wavelength of 1.53 μm). And an optical fiber grating can be used. The characteristics of such optical filters and optical fiber gratings are characterized in that they can be made more easily than the loss wavelength characteristics of non-excitation Er-doped optical fibers.

Further, the Er-doped optical fiber 10 for amplification
9, 109a and 109b include Er as shown in FIG.
An doped multi-core fiber or a conventional Er-doped optical fiber having one core can be used. In addition to the configuration in which the pumping light for pumping the Er-doped optical fiber 109 in FIGS. 1 and 6 is bidirectionally pumped, a unidirectional pumping method in which only one of the pumping light sources 108 and 112 is driven may be used. it can.

[0048]

As described above, according to the present invention, a first Er-doped optical fiber for performing bi-directional amplification is connected between a first optical circulator and a second optical circulator, and pumping of a predetermined wavelength is performed. Pumping means for supplying light to the Er-doped optical fiber from one direction or two directions, and connected between two ports of the second optical circulator, and a signal light from the signal light from the first Er-doped optical fiber; Since the optical components having the characteristic of attenuating or reflecting the specific wavelength band are provided, the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, and the optical S / N characteristics (optical signal / noise characteristics) And deterioration of optical crosstalk characteristics (optical crosstalk characteristics between signal lights of respective wavelengths) can be extremely reduced. Also,
Since the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, the specifications of the optical transmitter and the optical receiver can be made almost the same, and the mass productivity can be improved and the cost can be reduced.

Since the wavelengths of the upstream and downstream signal lights can be freely selected in the range of 1.53 μm to 1.56 μm, the flexibility and expandability of the system can be improved. Furthermore, since the amplification gains of the signal light on the upstream side and the signal light on the downstream side can be made substantially equal, the transmission distances on the upstream side and the downstream side can also be made substantially equal, and the system design becomes easy. In addition, since the excitation method is symmetrical on the up and down sides,
The noise figure characteristics on the up side and the down side can be made substantially equal.

Further, according to the present invention, a first Er-doped optical fiber and a second Er-doped optical fiber for performing bidirectional amplification are connected between the first optical circulator and the second optical circulator, Since the optical components for suppressing the gain deviation are provided between the first and second Er-doped optical fibers, the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, and the light S / S Deterioration of N characteristics (optical signal / noise characteristics) and optical crosstalk characteristics (optical crosstalk characteristics between signal lights of respective wavelengths) can be extremely reduced. Further, since the amplification gains of the signal lights of the respective wavelengths can be made substantially equal, the specifications of the optical transmitter and the optical receiver can be made substantially the same, and the mass productivity can be improved and the cost can be reduced. Further, the wavelengths of the upstream and downstream signal lights are set to 1.53 μm.
Since it can be freely selected in the range of m to 1.56 μm, flexibility and expandability of the system can be achieved. Furthermore, since the amplification gains of the signal light on the upstream side and the signal light on the downstream side can be made substantially equal, the transmission distances on the upstream side and the downstream side can also be made substantially equal, and the system design becomes easy. In addition, since the excitation method is symmetrical on the upstream side and the downstream side, the noise figure characteristics on the upstream side and the downstream side can be made substantially equal.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention.

FIG. 2 is a cross-sectional view showing a configuration of an Er-doped multi-core optical fiber that can be used as the Er-doped optical fiber of the present invention.

FIG. 3 is a characteristic diagram showing a loss wavelength characteristic of the Er-doped multi-core optical fiber of FIG. 2;

FIG. 4 is a circuit diagram showing a second embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention.

FIG. 5 is a circuit diagram showing a third embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention.

FIG. 6 is a circuit diagram showing a fourth embodiment of an Er-doped optical fiber amplifier for bidirectional transmission according to the present invention.

FIG. 7 is a circuit diagram showing a configuration of a conventional Er-doped optical fiber amplifier for bidirectional transmission.

FIG. 8 is a circuit diagram showing another configuration of a conventional Er-doped optical fiber amplifier for bidirectional transmission.

[Explanation of symbols]

101, 113, 140 Optical circulators 103, 118 Optical transmitters 105, 116 Optical receivers 106, 110, 136, 137 WDM couplers 108, 112 Excitation light sources 109, 109a, 109b, 114, 141 Er-doped optical fibers 115, 142, 143 WDM filter

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // G02B 6/00

Claims (5)

[Claims] A first and second optical transmitter for outputting wavelength-multiplexed first and second signal lights; and at least three ports, of which the first and second optical transmitters are connected by two ports. A first optical circulator connected to a first optical receiver for receiving the second signal light and receiving the second signal light to another port; a second light having at least four ports A circulator, an Er-doped optical fiber connected between the first optical circulator and the second optical circulator, for performing bidirectional amplification, and a pumping light of a predetermined wavelength to the Er-doped optical fiber in one direction or Pumping means for providing from both directions; gain adjusting means connected between two ports of the second optical circulator for attenuating or reflecting a specific wavelength band of the signal light from the Er-doped optical fiber; A WDM filter connected to another port of the second optical circulator and connected to the second optical transmitter and a second optical receiver for receiving the first signal light; An Er-doped optical fiber amplifier for bidirectional transmission, characterized in that: 2. The Er-doped optical fiber amplifier for bidirectional transmission according to claim 1, wherein said gain adjusting means is a non-pumped Er-doped optical fiber, an interference film optical filter, or an optical fiber grating. 3. The first optical circulator includes a second optical circulator to which the first optical transmitter and the first optical receiver are connected.
2. The WDM filter according to claim 1, wherein said WDM filter is connected to a gain adjusting means having a characteristic of attenuating or reflecting a specific wavelength band of the signal light from said second optical transmitter.
An Er-doped optical fiber amplifier for bidirectional transmission according to claim 1. 4. A first and second optical transmitter for outputting wavelength-multiplexed first and second signal lights, and at least three ports, of which two ports are the first and second optical transmitters. A first optical circulator connected to a first optical receiver for receiving the second signal light and receiving the second signal light to another port; a second light having at least four ports A first circulator connected between the first optical circulator and the second optical circulator for performing bidirectional amplification;
A first optical fiber between the first and second optical circulators;
A gain adjusting means connected to the Er-doped optical fiber for attenuating or reflecting a specific wavelength band in the signal light; and a bi-directional amplifier connected between the first and second optical circulators. A second Er-doped optical fiber, a pumping means for supplying pumping light of a predetermined wavelength to the first and second Er-doped optical fibers in one direction or two directions, and connected to the second optical circulator. And the second optical transmitter, and a second optical transmitter for receiving the first signal light.
An Er-doped optical fiber amplifier for bidirectional transmission, comprising: a WDM filter to which the optical receiver is connected. 5. The Er-doped optical fiber amplifier for bidirectional transmission according to claim 4, wherein said gain adjusting means is a non-pumped Er-doped optical fiber, an interference film optical filter, or an optical fiber grating.