JPH06342952A - Optical fiber amplifier and optical fiber transmission system - Google Patents

Abstract

PURPOSE:To provide a bidirectional optical fiber amplifier which can bidirectionally amplify a plurality of signal lights having different wavelength bands to propagating in an optical fiber in an optical transmission system in which the fiber is used as a transmission line. CONSTITUTION:An optical fiber amplifier 10 comprises an exciting semiconductor laser 11 for a wavelength of 1.48mum, a wavelength multiplexing/ demultiplexing unit 12, and an erbium-doped optical fiber 13. Two optical multiplexing units 20, 30 respectively have an optical circulator 21, an optical filter 22 having a wavelength transmitting band of 1.54mum or less, an optical fiber 23 having a wavelength transmitting band of 1.55mum or more, an optical multiplexer 24, an optical circulator 31, an optical filter 32 having a wavelength transmitting band of 1.54mum or less, an optical fiber 33 having a wavelength transmitting band of 1.55mum, and an optical multiplexer 34.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system using an optical fiber as a transmission line, and is capable of amplifying a plurality of signal lights of different wavelength bands which propagate bidirectionally through the optical fiber. Optical fiber amplifier of.

[0002]

2. Description of the Related Art Conventionally, an optical fiber amplifier includes a rare earth-doped optical fiber, a pumping semiconductor laser light source for optically pumping a rare earth element, and an optical multiplexer for coupling the pumping semiconductor laser light to the rare earth-doped optical fiber. , And an optical isolator. When the input signal and the pumping light are wavelength-multiplexed and input to the rare earth-doped optical fiber amplifier, the rare earth-doped optical fiber is brought into an inverted distribution state by the pumping light, and as a result, the signal light is amplified. Then, the amplified signal light is emitted through the optical filter. Here, when erbium is used as the rare earth, the excitation light is usually 0.98 μm.
A band or 1.45-1.5 μm band laser light is used. The optical isolator has a problem that when the gain of the optical fiber amplifier is high, the feedback of light occurs due to the reflectance remaining inside or outside the optical fiber amplifier, and laser oscillation occurs.
It is used for suppressing reflected light in order to solve the problem that noise increases even if laser oscillation is not caused by similar light feedback. The optical isolator has a characteristic that it transmits light traveling in one direction by reciprocal action, but blocks light traveling in the opposite direction, and can prevent the return of light to the optical fiber amplifier. The optical amplifier used can only amplify signals traveling in one direction.

Hitherto, as a bidirectional optical amplifier,
As shown in FIG. 12, a configuration has been known in which an optical isolator that determines the direction of amplification is omitted from the configuration of a conventional optical amplifier (see Document 1 below).

In FIG. 12, 101 and 105 are optical fibers, 102 is an optical wavelength multiplexer, 103 is a pumping semiconductor laser, and 104 is a rare earth doped fiber. In this configuration, the above-mentioned phenomenon of deterioration of the transmission characteristics occurs due to the residual reflectance at the connecting portion and the connector portion of the optical fiber transmission line (Reference 1: J. Haugen et al., "Bidirectional Transmision").
sion at 622Mb / s Utilizing Erbium-Doped Fiber Ampli
fiers ", IEEE Photonics Technology Letters, vol.4,
p913 (1992)).

Further, there are the following documents as bidirectional optical amplifiers and systems using them. JP-A-5-224253, JP-A-5-227102, and 1993 Spring Conference of the Institute of Electronics, Information and Communication Engineers C-198, 199 (4-234, page 235), B-884 (page 4-125).

[0006]

However, in the conventional example of the above-mentioned Document 1, the above-mentioned problems are caused by various residual reflectances inherent in the connecting points of the optical fibers and the transmission lines from the optical parts. As described above, there is basically a problem of instability in which noise increases or an optical amplifier oscillates with a laser, and it is difficult to apply to a general transmission line. This problem is obvious from the fact that the optical isolator is always used even in the one-way optical amplifier configuration, as seen in Reference 2 (Reference 2: Shimada et al., “Opening a new era of optical communication systems”). Optical fiber amplifiers, high-speed and long-distance transmission experiments one after another ”, NIKKEI ELECTRONICS, p43 (1991.7.
1)).

Therefore, the present invention provides a bidirectional optical fiber amplifier capable of bidirectionally amplifying a plurality of signal lights having different wavelength bands propagating through an optical fiber.

[0008]

In order to achieve the above object, the optical fiber amplifier of the present invention is a so-called optical fiber amplifier section comprising a rare earth-doped optical fiber and means for optically pumping the rare earth element. And an optical filter having a wavelength band (A) and a wavelength band (B) for selecting different wavelength transmission bands from the gain bands of the optical fiber amplifying unit, respectively, in two terminals of the three-terminal optical circulator. And an optical combiner configured by optically connecting an optical multiplexer via both ends of an optical fiber of the optical amplifier, and terminals different from the two terminals of the optical circulator of the optical combiner, respectively. The light of the wavelength band (A) incident from the optical multiplexer of the optical combining unit connected to the end different from the optical amplifying unit of the optical fiber amplifying unit is configured by connecting the optical combining unit via A wavelength band that is amplified and emitted from the optical combiner of the optical combiner connected to the other end of the optical fiber amplifier, and is incident from the optical combiner of the optical combiner connected to the other end of the optical fiber amplifier. The light of (B) is amplified and emitted in the opposite direction to the light of the wavelength band (A).

According to another aspect of the optical fiber amplifier of the present invention, a rare earth element-doped optical fiber, an optical amplifying section composed of a means for optically exciting the rare earth element, and a first signal light are incident, A first circulator that emits two signal lights, a second circulator that inputs the second signal light, and a second circulator that emits the first signal light, and the first and second signal lights A first photosynthesis branching device, and the first and second
At least a second optical combining / branching device for emitting the signal light of the first and second optical combining / branching devices, wherein the first and second optical combining / branching devices include a wavelength band (A) and a wavelength band (B) in the gain band of the optical amplification unit. In the first three-terminal optical circulator, one terminal of the first three-terminal optical circulator is connected to the terminal for entering the wavelength band (the former) of the first photosynthetic branching device, and the second three-terminal One terminal of the optical circulator is connected to a terminal for entering the wavelength band (B) of the first optical combining / branching device, and the other one terminal of the first three-terminal optical circulator is connected to the first optical circulator. It is connected to the terminal for emitting the wavelength band (B) of the photosynthesis branching device, and the other one terminal of the second three-terminal optical circulator is a terminal for emitting the wavelength band (A) of the first photosynthesis branching device. Is connected to both ends of the optical fiber of the optical amplifying section, respectively. Beauty connects the different terminals and the two terminals of the second photosynthetic splitter, said first
The light in the wavelength band (A) incident from a terminal different from the two terminals of the three-terminal optical circulator of
The light is amplified and emitted from the terminal optical circulator,
Wavelength band incident from the 3-terminal optical circulator (Otsu)
Light is amplified and emitted in the opposite direction to the light in the wavelength band (instep).

[0010]

In the present invention, in order to perform bidirectional optical amplification, 2
By assigning different wavelength bands as signal bands in the respective transmission directions, a feedback loop due to residual reflectance cannot be performed for each signal wavelength. Furthermore, so that the signal lights in both directions with different wavelength bands are incident on the optical amplification section composed of one optical amplifier,
An optical circulator or an optical wavelength synthesizing / branching device constitutes a light synthesizing unit. This enables bidirectional amplification of signal light without increasing noise or causing laser oscillation.

Further, in the rare earth-doped optical fiber of the amplifying section, the signal intensity distributions proceeding in respective exponentially increasing directions are different so as to complement each other, so that the influence of interference between signals is suppressed to a small level. You can hold it.

[0012]

【Example】

(Embodiment 1) Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of the first embodiment.

In FIG. 1, reference numeral 10 is an optical amplifier, 20, and 3.
Reference numeral 0 is a photosynthesis unit, 11 is a semiconductor laser for excitation having a wavelength of 1.48 μm, and 12 is wavelength synthesis. A branching device, 13 is an erbium-doped optical fiber, 21 and 31 are optical circulators, 22 and 32 are optical filters with a wavelength transmission band of 1.54 μm or less, 23 and 33 are optical filters with a wavelength transmission band of 1.55 μm or more, and 24 and 34. Is an optical multiplexer / demultiplexer (optical coupler),
Reference numerals 25 and 35 are optical fibers. 41 is an optical fiber 25
To the optical fiber 35, 42 is the amplified first signal light emitted from the optical fiber 35, and 51 is the optical fiber 35.
Is the second signal light incident on the optical fiber 25, and 52 is the amplified second signal light emitted from the optical fiber 25.

Here, the wavelength characteristics of the gain of the erbium-doped optical fiber 13 are shown in FIG. As shown in the figure, the optical fiber has a gain characteristic with respect to the wavelength of incident light.

Next, FIG. 3 shows the wavelength characteristics of the transmittance of the optical filters 22, 23, 32 and 33. Optical filters 21, 32
Is a filter that transmits a wavelength of 1.54 μm or less, and the optical filters 23 and 33 are a filter that transmits a wavelength of 1.54 μm or more. As shown in FIG.
The optical filters 22, 23, 32, 33 shown in FIG. 1 are indicated by arrows in FIG. The long arrow means that the light having a wavelength of 1.54 μm or more is transmitted, and the short arrow means that the light having a wavelength of 1.54 μm or less is transmitted.

FIG. 4 shows the wavelength characteristics of the gain for light traveling to the left and the gain for light traveling to the right of the optical amplifier of the present invention obtained by the action of the wavelength filters 22, 23, 32, 33. That is, FIG. 4 corresponds to FIG. 2 and FIG.
It is expressed by multiplication. That is, the optical filters 22, 3
The gain characteristic of the light passing through 2 is the wavelength band represented by B in FIG. 4, and the gain characteristic of the light passing through the optical filters 23 and 33 is the wavelength band (step A).

The function of the optical circulator is described in, for example, the Institute of Electronics and Communication Engineers: Optical and Quantum Electronics Research Group report (OQE 79-20, "Small optical circulator for optical fiber communication)," (OQE 78-149, "Optical circulator has It has been reported in "An attempt to remove the polarization dependence"), but a brief explanation is that the light is rotated only in a fixed direction.
The optical multiplexer / demultiplexer 24 to the optical filter 2
It is assumed that light having a wavelength of 1.54 μm or less that has passed through 1 enters the optical circulator 21. This light can enter the wavelength combining / branching device 12.

However, even if the light having the wavelength of 1.54 μm or less that has passed through the optical filter 32 from the optical multiplexer / demultiplexer 34 enters the optical circulator 31, this light does not enter the optical fiber 13. This is because the optical circulator 31 rotates the light only in a fixed direction, and this light is cut by the circulator 31.

The optical fiber amplifier 10 has a wavelength of 1.48 μm.
Pumping semiconductor laser 11, wavelength synthesis. It is composed of a branching device 12 and an erbium-doped optical fiber 13. Further, the two light combining units 20 and 30 are respectively provided in the optical circulator 2.
1, an optical filter 22 having a wavelength transmission band of 1.54 μm or less,
Optical filter 23 with wavelength transmission band of 1.55 μm or more, optical multiplexer / demultiplexer (optical coupler) 24 and optical circulator 31, optical filter 32 with wavelength transmission band of 1.54 μm or less, optical filter 33 with wavelength transmission band of 1.55 μm or more , An optical multiplexer / demultiplexer (optical coupler) 34, and each portion is optically coupled by using a space or an optical fiber.

Next, the case where the signal light enters the optical fiber amplifier will be described. Now, the signal light 41 having a wavelength of 1.535 μm enters from the left side of the optical fiber 25. The light 41 is split into two optical paths by the optical multiplexer / demultiplexer 24, and then passes through the optical filters 22 and 23 and enters the optical circulator 21. The optical filter 22 is 1.54 μm
Since the following light is transmitted, the incident signal light 41 passes through the filter 22. However, the incident signal light 41 traveling in the direction of the optical filter 23 is
It is cut from the characteristics of 3 (explained in FIG. 3).

Of the incident signal light 41 that has proceeded to the optical circulator 21, only the signal light that has passed through the optical filter 22 enters the erbium optical fiber 13 due to the function of the optical circulator as described above. Even if the incident signal light 41 passes through the optical filter 23, it does not proceed to the optical amplification section 10 due to the function of the optical circulator.

Therefore, the light 4 transmitted through the optical filter 22
Only 1 goes to the wavelength synthesizing / branching device 12 of the optical amplifying unit 10 and is amplified.

The amplified signal light 41 is emitted only toward the optical filter 32 by the optical circulator 31 arranged at the other end, and is not emitted toward the optical filter 33. The wavelength of this signal light 41 is within the band of the optical filter 32 (1.54 μm
Since the following), the optical multiplexer / demultiplexer 3 is not affected by the loss.
4 and enters the optical fiber 35 through the optical multiplexer / demultiplexer 34.
Is amplified to the right of and is emitted as the amplified signal light 42.

When the signal light 42 once emitted enters the optical multiplexer / demultiplexer 34 from the right side of the optical multiplexer / demultiplexer 34 again due to some external reflection, this light is emitted from the optical filter 3
Although it passes through the optical circulator 31, it does not proceed to the optical fiber 13 even if it enters the optical circulator 31, so that it does not affect the characteristics such as noise of the amplifier. Further, even when the light travels to the optical filter 33, this light is outside the band of the filter 33, so that it is sufficiently attenuated and then enters the optical amplifying section 10, so that it is not amplified and does not affect the characteristics such as noise of the amplifier.

Similarly, the signal light 51 having a wavelength of 1.555 μm entering from the right side of the optical fiber 35 is split into two optical paths by the optical multiplexer / demultiplexer 34, and then passes through the optical filters 32 and 33, respectively. And enters the optical circulator 31. At this time, due to the function of the optical circulator, only the signal light that has passed through the 33 optical filter is erbium optical fiber 1
It is incident on 3 and is amplified. The amplified signal light 51 is transmitted to the optical filter 2 by the optical circulator 21 arranged at the other end.
The light is emitted only in the direction of 3. Since the wavelength of this signal light 51 is within the band of the optical filter 23 (band of 1.54 μm or more),
The amplified signal light 52 is emitted to the left side of the optical fiber 25 through the optical multiplexer / demultiplexer 24 without being affected by the loss.

The optical multiplexer / demultiplexer 2 is provided by reflection from the outside.
The signal light once emitted to the left side of 4 is the optical multiplexer / demultiplexer 2 again
4 enters from the left side, the light passing through the optical filter 22 is out of the band of the optical filter 22 and thus is sufficiently attenuated, then passes through the optical circulator 21 and enters the erbium optical fiber 13, so that the noise of the amplifier is reduced. It does not affect the characteristics such as. Further, the light passing through the optical filter 23 does not travel in the direction of the erbium optical fiber 13 due to the function of the optical circulator 21, and does not affect the characteristics such as noise of the amplifier.

Now, let us consider the influence of the return of light when there is reflection outside the optical amplifier. If the wavelength of the signal light is l, the external power reflectance is R, the power amplification of the erbium optical fiber is G, and the power transmittance of the optical filter is T1 (l), T2 (l), the signal light due to reflection is Gain = (R x G x T1 (l) x T
2 (l)) 2 is amplified.

The condition of gain = 1 is the threshold condition for the erbium optical fiber to start laser oscillation. Now R = 0.00
If 1, G = 10000, T1 (l) = 1, T2 (l) = 0.01, gain =
It is 0.01, which is sufficiently smaller than the laser oscillation condition. In this case, while having a bidirectional optical amplification function,
It can be seen that the external reflectance does not affect the characteristics of the amplifier such as noise.

On the other hand, in the configuration of FIG. 12 used in the description of the conventional example, it was found that T1 = T2 = 1, the gain = 100, the laser oscillation condition was exceeded, and the influence of external reflection was significant. It is quantitatively understood that the present invention is very effective in comparison with the conventional example.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS. In the following description, the same parts as those described above will be designated by the same reference numerals and the description thereof will be omitted. FIG. 5 is a schematic diagram showing the configuration of the second embodiment. The difference from the first embodiment is that optical wavelength combiners / splitters are used for 26 and 36 in FIG. The light wavelength combining / branching devices 26 and 36 are a kind of wavelength filters and guide a certain fixed wavelength in a predetermined direction. Here, the optical wavelength combining / branching device 26 is
The wavelength of 1.54 μm or less is guided to the optical filter 22, and 1.54 μm
It has a function of guiding the above wavelengths to the optical filter 23.
In FIG. 5, since the wavelength of the first signal light 41 is 1.535 μm, the light travels only in the direction of the optical filter 22 by the combining / branching device 26. In the first embodiment, the light entering from the optical fiber first enters the optical combiner / splitter 24 and then enters the optical filters 22 and 23. Therefore, in comparison with this, the second embodiment combines and splits the optical wavelengths. Since the device is used, the light utilization efficiency can be increased.

The optical fiber amplifier 10 is composed of 11, 12 and 13 as in the first embodiment. Further, the two photosynthesis sections 20 and 30 are respectively 21, 22, 23 and 26.
, 31, 32, 33, and 36.

FIG. 6A shows a wavelength distribution of signal intensity when different digital signals of three wavelengths are added as the signal light 41 and an analog signal of one wavelength is added as the signal light 51 as a configuration example of the signal light. .

In general, an analog signal requires a higher signal strength than a digital signal in order to obtain a sufficient signal noise ratio. Therefore, in consideration of the shift of the gain peak toward the long wavelength side due to the gain saturation of the optical amplification fiber. , Set to the long wavelength side.
This is because, as shown in FIG. 6B, there is a difference in gain between the incident light having a large signal intensity and the incident light having a small signal intensity. Looking at the gain distribution of light with a large signal strength, it is clear that the long wavelength side has a larger gain than the low wavelength side. Therefore, since the analog signal requires a large signal strength, it is set on the long wavelength side.

Further, since the digital signal does not require such a high signal strength, the interfering effect between the signals hardly occurs even in the bidirectional amplification action by one optical amplifying section.
This is because there is a considerable difference between the signal strength of the analog signal traveling to the right as shown in FIG. 6C and the magnitude of the digital signal traveling to the left.

By setting the signal in this way,
It is possible to transmit analog signals with low noise and low distortion and simultaneously transmit digital signals bidirectionally. In such a signal wavelength configuration, optical broadcasting from a master station such as CATV to a home terminal is collectively broadcasted such as video transmission using the analog signal described above, and a data signal from the home terminal is performed using the digital signal described above. Used in some cases.

FIG. 7 shows the wavelength distribution of the signal light when a different digital signal having two wavelengths is used as an LED (light emitting diode) as the signal light 41 and an analog signal having one wavelength is added as the signal light 51. This is shown as a configuration example.

When a plurality of signals are transmitted by LD (semiconductor laser) light, beats are generated due to the frequency difference of light and become noise components. However, by using LEDs as a plurality of signal light sources, beats are generated. Can be suppressed. Also in this case, similarly to the case described with reference to FIG. 6A, since an analog signal generally requires a higher signal strength than a digital signal,
It is set to the long wavelength side in consideration of the shift of the gain peak to the long wavelength side due to the gain saturation of the optical amplification fiber.

By setting the signal in this way,
It is possible to transmit analog signals with low noise and low distortion and simultaneously transmit digital signals bidirectionally.

(Third Embodiment) The third embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 8 is a schematic diagram showing the structure of the third embodiment. In the following description, the same parts as those already described are designated by the same reference numerals.

The optical fiber amplifier 10 is composed of an optical combiner 20, a pumping semiconductor laser 11 having a wavelength of 1.48 μm, a wavelength combiner / splitter 12, and an erbium-doped optical fiber 13 as in the first embodiment. The optical combiner 20 includes optical circulators 21 and 31, wavelength combiners / splitters 26 and 36,
It is composed of optical fibers 25 and 35. Reference numeral 41 is the first signal light incident on the optical fiber 25, 42 is the amplified first signal light emitted from the optical fiber 35, 51 is the second signal light incident on the optical fiber 35, and 52 is the optical fiber 25. It is the amplified second signal light emitted from. The optical circulators 21 and 31 are the same as those in the first embodiment and the wavelength synthesizing / branching device 23.
And 36 have characteristics equivalent to those of the second embodiment.

Next, the case where signal light is incident on this optical fiber amplifier will be described. Now, the signal light 41 having a wavelength of 1.535 μm enters from the left side of the optical fiber 25. When this light 41 enters the optical circulator 21, the optical circulator 21 functions as described above to operate the optical fiber 25.
Only the signal light from is incident on the wavelength synthesizing / branching device 26, is multiplexed with the signal light 51 having a wavelength of 1.560 μm, and is then incident on the optical fiber amplifier 10. The wavelength synthesizing / branching device 26 has the characteristics of FIG. 3, so that the wavelengths of 1.535 μm and 1.56 μm
The signal light of 0 μm can be combined with low loss.

The amplified signal light 41 is emitted only in the direction of the optical circulator 31 by the wavelength synthesizing / branching device 36 arranged at the other end, and is not emitted in the direction of the optical circulator 21. When this signal light 41 enters the optical circulator 31, only the signal light amplified by the function of the optical circulator is transmitted toward the optical fiber 31 and emitted as the amplified signal light 42.

Similarly, the signal light 51 having a wavelength of 1.560 μm, which is incident from the right side of the optical fiber 35, is incident on the optical circulator, and then is transmitted to the optical multiplexer / demultiplexer 26 at 1.535 μm.
Signal light 41 and is incident on the optical amplifier 10 and amplified. The amplified signal light 51 is transmitted by the wavelength multiplexer / splitter 36 arranged at the other end of the signal light 51 having a wavelength of 1.560 μm.
Only the light passes through in the direction of the optical circulator 21, and is emitted as signal light 52 amplified by the optical circulator 21 to the left side of the optical fiber 25.

The gain in the case of a small signal input is 30 dB or more, but since there is an optical isolator with a return loss of -60 dB or less at the EDF end, it does not affect the characteristics such as noise of the amplifier. As described above, in this embodiment, since the optical isolator is used at both ends of the EDF to remove the influence of reflection, when transmitting an analog signal, there is a remarkable effect in preventing characteristic deterioration.

Further, as a multiplexer / branching device for the signal lights, all wavelength synthesizers have been used conventionally. This is one
Since only the branching ratio of signal light of about -20 to -30 dB can be obtained,
In particular, there have been problems such as deterioration of noise characteristics during analog signal transmission, and large optical loss due to cascade connection of a plurality of wavelength combiners in order to obtain a sufficient branching ratio.

However, since the optical circulator used in this embodiment shown in FIG. 8 has a very small leaked light between the terminals of -70 dB or less, it is used in the input / output portion with the optical transmission line (optical fiber). By replacing it with a wavelength synthesizer,
An excellent characteristic is obtained in that the structure is simplified and a branching ratio of 70 dB or more can be obtained.

FIG. 9 shows an erbium-doped optical fiber (E
DF) shows the population inversion coefficient at each point. When bidirectional signal light is incident from both ends of the EDF, the other signal light is most increased in the vicinity of the incident portion of one signal light, so that the population inversion is lowered and the noise figure (NF) is deteriorated.

On the other hand, in the structure of the third embodiment, E
Since the signal light is incident on the DF from the same direction, both populations of the signal light are very small at the signal light incidence portion, so that the population inversion does not decrease and the NF does not easily deteriorate. From this, it can be seen that the configuration of this bidirectional optical amplifier is effective particularly in a system in which NF is important.

(Embodiment 4) FIG. 6A shows the wavelength distribution of the signal intensity when different digital signals of three wavelengths are added as the signal light 41 and an analog signal of one wavelength is added as the signal light 51. This is shown as a configuration example of.

As described above, since the analog signal requires a higher signal strength than the digital signal in order to obtain a sufficient signal noise ratio, the shift of the gain peak to the long wavelength side due to the gain saturation of the optical amplifying fiber is required. Considering this, it is set to the long wavelength side. This is because, as shown in FIG. 6B, there is a difference in gain between the incident light having a large signal intensity and the incident light having a small signal intensity. Looking at the gain distribution of light with a large signal strength, it is clear that the long wavelength side has a larger gain than the low wavelength side. Therefore, since the analog signal requires a large signal strength, it is set on the long wavelength side.

Since the digital signal does not require such high signal strength, the input light quantity may be small. Therefore, even in the bidirectional amplifying action by one optical amplifying unit, the interfering effect between signals hardly occurs. This is because there is a considerable difference in magnitude between the analog signal strength and the digital signal as shown in FIG.

However, when the input light amount of the signal light on the long wavelength side is large and therefore the saturation occurs, the signal on the short wavelength side also saturates as shown in FIG. 10 (a), and the gain decreases. On the contrary, FIG. 10B shows a case where the input light amount of the signal light on the short wavelength side is large, but it can be seen from FIG. 10A that gain saturation is less likely to occur. This is because the light on the short wavelength side of the large input acts as excitation light for the light on the long wavelength side and is absorbed.

It is necessary that the transmission distortion characteristics of the analog signal be sufficiently small and that interference between signals does not occur, but under the condition that interference of gain characteristics does not occur, interference of distortion characteristics hardly occurs. This is because the distortion generated in the optical fiber amplifier is determined by the gain tilt. Figure 1
FIG. 1 (a) is a diagram showing the CSO distortion of the signal light on the long wavelength side with respect to the input light amount of the signal light on the short wavelength side.
Comparing with (a), it can be seen that the distortion characteristics are also beginning to deteriorate under the same conditions as when the gain decreases. FIG. 11 (b) shows the CSO distortion characteristic with respect to the input light amount on the long wavelength side when the signal light on the short wavelength side is analog-modulated.
Similarly, it can be seen that the distortion characteristics are more likely to deteriorate than in FIG.

The tolerance for distortion characteristic deterioration is strict for analog signals, but relatively lenient for digital signal transmission characteristics. Therefore, if an analog signal is assigned to the signal light on the long wavelength side and a digital signal is assigned to the signal light on the short wavelength side, good transmission characteristics can be obtained, and the number of wavelength-multiplexed digital signals increases or decreases, and the input light amount is increased. Stable analog transmission characteristics are assured even when the value changes.

By setting the signals in this way,
It is possible to transmit analog signals with low noise and low distortion and simultaneously transmit digital signals bidirectionally. In such a signal wavelength configuration, optical broadcasting from a master station such as CATV to a home terminal is collectively broadcast by video transmission, etc. by the analog signal described above, and a data signal from the home terminal is made by the digital signal described above. Used in some cases.

In the present embodiment, as the signal light of the wavelength band (A), only the analog signal light of one wavelength is shown, but at the same time, other wavelengths in the wavelength band (A) are used and other It is possible to transmit a plurality of analog or digital signal lights. Further, similarly, it goes without saying that the signal light of a plurality of wavelengths can be transmitted as the signal light of the wavelength band (B).

In the present embodiment, the 1.5-micron band optical amplifier to which the rare earth element erbium is added has been described, but the present invention is also applicable to optical amplifiers of different wavelength bands using other rare earth elements such as neodymium and praseodymium. The above effect can be obtained, and the constituent materials thereof are not limited.

[0058]

As described above, according to the present invention,
It is possible to provide a high-performance bidirectional optical fiber amplifier that amplifies a plurality of different signal lights entering the optical fiber from two different directions with a simple configuration using one optical fiber amplifier.

Further, according to the present invention, a high-performance bidirectional optical fiber amplifier which amplifies a plurality of different signal lights incident on the optical fiber from the same direction with a simple structure using one optical fiber amplifier. There is an effect that can be provided.

Further, according to the present invention, there is an effect that a stable analog transmission characteristic can be provided even when the number of channels of a digital signal increases or decreases.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the wavelength dependence of the amplification gain of an erbium-doped optical fiber in the example of the present invention.

FIG. 3 is a diagram showing the wavelength dependence of the optical filter transmittance in the example of the present invention.

FIG. 4 is a diagram showing the wavelength dependence of gain after inserting an optical filter in an example of the present invention.

FIG. 5 is a configuration diagram of a second embodiment of the present invention.

FIG. 6 is a configuration diagram of an optical signal wavelength according to a second embodiment of the present invention.

FIG. 7 is a configuration diagram of an optical signal wavelength according to a second embodiment of the present invention.

FIG. 8 is a configuration diagram of a third embodiment of the present invention.

FIG. 9 is a diagram showing a population inversion state in an erbium-doped optical fiber according to a third embodiment of the invention.

FIG. 10 is a diagram showing a gain characteristic in the fourth embodiment of the present invention.

FIG. 11 is a diagram showing CSO strain characteristics in the fourth example of the present invention.

FIG. 12 is a block diagram of a conventional bidirectional optical amplifier.

[Explanation of symbols] 10 optical amplifier 11 pumping semiconductor laser 12 wavelength synthesizer 13 erbium-doped optical fiber 14 optical isolator 15 optical isolator 20 optical synthesizer 21 optical circulator 22 optical filter of wavelength band (step A) 23 wavelength band (B) Optical filter 24 Optical multiplexer 25 Optical fiber 26 Wavelength combiner / splitter 30 Optical combiner 31 Optical circulator 32 Optical filter in wavelength band (step A) 33 Optical filter in wavelength band (Otsu) 34 Optical multiplexer 35 Optical fiber 36 Wavelength combiner・ Switch

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04B 10/24 10/16

Claims (9)

[Claims] 1. A light amplifying section comprising a rare earth-doped optical fiber, means for optically exciting the rare earth element, and a gain of the light amplifying section at two terminals of a three-terminal optical circulator. At least a light combining unit configured by optically connecting an optical multiplexer via an optical filter of a wavelength band (A) and a wavelength band (B) for selecting different wavelength transmission bands from the bands The optical combining section is connected to both ends of the optical fiber of the optical amplifying section via terminals different from the two terminals of the optical circulator of the optical combining section, respectively, and to different ends of the optical amplifying section. The light in the wavelength band (A) incident from the connected optical combiner of the optical combiner is amplified and emitted from the optical combiner of the optical combiner connected to the other end of the optical amplifier, and the optical amplifier is output. Connected to the other end of Optical fiber amplifier light is characterized in that the light in the wavelength band (MIG) is configured to emit is amplified in the direction opposite to the wavelength band emitted from the optical multiplexer coupling section (Otsu). 2. The optical fiber amplifier according to claim 1, wherein the optical multiplexer is an optical wavelength combiner / splitter. 3. An optical fiber transmission line composed of optical fibers connected to both sides of the optical fiber amplifier according to claim 1 or 2 via an optical multiplexer of the optical combiner,
An optical fiber transmission system, characterized in that at least two signal lights having respective wavelength bands of the above (A) and (B) are bidirectionally transmitted to each other. 4. The optical fiber transmission system according to claim 3, wherein at least one analog-modulated signal light is transmitted in a wavelength band on the long wavelength side of the wavelength bands of (A) and (B). A two-way optical fiber transmission system that transmits a plurality of digitally modulated signal lights in the wavelength band on the shorter wavelength side of the wavelength bands of (A) and (B). 5. The optical fiber transmission system according to claim 4, wherein a plurality of digitally modulated signal lights in the wavelength band on the shorter wavelength side of the wavelength bands of (A) and (B) are emitted. An optical fiber transmission system characterized by using a diode as a light source. 6. An optical amplifying section composed of a rare earth element-doped optical fiber and a means for optically exciting the rare earth element, a first signal light beam, and a second signal light beam. A circulator, a second circulator that receives the second signal light and emits the first signal light, a first photosynthesis splitter that receives the first and second signal lights, At least a second optical combining / branching device that emits first and second signal lights is provided, and the first and second optical combining / branching devices have a wavelength band (instep A) within a gain band of the optical amplification unit. The wavelength band (B) is to be multiplexed / demultiplexed, and one terminal of the first three-terminal optical circulator is connected to a terminal for entering the wavelength band (A) of the first photosynthetic branching device, One of the terminals of the second three-terminal optical circulator is a wave of the first optical combining / branching device. The other terminal of the first three-terminal optical circulator is connected to the terminal for inputting the band (B), and the terminal for outputting the wavelength band (B) of the first photosynthetic branching device is connected to the terminal. The other one terminal of the two 3-terminal optical circulator is connected to the terminal for emitting the wavelength band (the former) of the first optical combining / branching device, and is connected to both ends of the optical fiber of the optical amplifying unit, respectively. Terminals different from the two terminals of the first and second optical combining / branching devices are connected to each other, and a wavelength band (A) incident from a terminal different from the terminals of the two terminals of the first three-terminal optical circulator is connected. Light is amplified and emitted from the second three-terminal optical circulator, and light in the wavelength band (B) incident from the second three-terminal optical circulator is amplified in the opposite direction to the light in the wavelength band (A). Configured to emit And an optical fiber amplifier. 7. An optical fiber transmission line composed of optical fibers connected to both sides of the optical fiber amplifier according to claim 6 via an optical multiplexer of the optical combiner is used to An optical fiber transmission system, characterized in that at least two signal lights each having a wavelength band of (B) are transmitted bidirectionally. 8. The optical fiber transmission system according to claim 6, wherein at least one analog-modulated signal light is transmitted in the wavelength band on the long wavelength side of the wavelength bands of (A) and (B). A two-way optical fiber transmission system that transmits a plurality of digitally modulated signal lights in the wavelength band on the shorter wavelength side of the wavelength bands of (A) and (B). 9. The optical fiber transmission system according to claim 6, wherein a plurality of digitally modulated signal lights in the wavelength band on the shorter wavelength side of the wavelength bands of (A) and (B) are emitted. An optical fiber transmission system characterized by using a diode as a light source.