JPH07212315A - Light amplifier - Google Patents

Abstract

PURPOSE:To maintain gain or output power to a desired fixed value and at the same time, to match a gain peak wavelength with a signal light wavelength, by providing a light amplifier part control means and a variable light attenuation means within a light amplifier. CONSTITUTION:A light amplifier 11 is used in a light amplifier multistage repeating transmission system where a subordinate connection is performed for plural light amplifiers at multistage between transmission and reception terminal stations 14 along an optical fiber 13, has a light amplifier part 25 composed of a rare earth dope fiber, multiplexes exciting light with input signal light, amplifies it and derives output signal light. In this case, the light amplifier 11 is provided with a light amplifier part control means 31 for matching the gain peak wavelength in the system with the wavelengths of input and output signal light and a variable light attenuation means 32 adjusting the level of the signal light power from the light amplifier part 25 and making output signal light to a prescribed signal light power level. In the means 31, the gain peak wavelength can be matched with the signal light wavelength by making exciting light of a proper level incident.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier, and more particularly to an optical amplifier suitable for application to an optical amplifier repeater which constitutes an optical amplification multistage repeater transmission system. In recent years, in order to put an ultra-long-distance optical communication system into practical use, a plurality of optical amplification repeaters, each of which functions as an optical amplifier, are configured by cascade connection in multiple stages along an optical fiber. R & D is being actively pursued.

The most important issue in the research and development of such an optical amplification multistage repeater transmission system is to secure high transmission quality. A signal-to-noise strength ratio (SNR) is used as a measure for evaluating the transmission quality.
o) is the most common. The present invention describes an optical amplifier intended to improve the SNR.

[0003]

2. Description of the Related Art FIG. 24 is a diagram showing a general configuration of an optical amplification multistage repeater transmission system. In the figure, an optical amplification multistage repeater transmission system 10 comprises a long-distance optical fiber 13 laid on the land or seabed between both end stations (A, B) 14.
And a plurality of optical amplification repeaters 12 that are inserted into the optical fiber 13 at predetermined intervals and that repeat optical signals in multiple stages.
Each optical amplification repeater 12 is an optical amplifier 1 to which the present invention is applied.
1 is included. The optical signal in which the terminal station (A) 14 is the transmitting terminal station and the terminal station (B) 14 is the receiving terminal station is transmitted through the optical amplifier 11 shown in the upper side of the figure, and the terminal station (B) 1
An optical signal 4 of which is a transmitting terminal station and the terminal station (A) 14 is a receiving terminal station is transmitted via an optical amplifier 11 shown at the lower side of the figure. The optical signal thus attenuated in the optical fiber is transmitted to the terminal station 14 on the receiving side while being amplified by each optical amplifier 11.

FIG. 25 is a diagram showing a conventional optical amplifier using a rare earth-doped fiber. In the figure, the input signal light received from the optical fiber 13 on the input side reaches the multiplexer 22 through the optical coupler 21, where the weak input signal light is mixed with the pump light and input to the optical amplifier 25. It When a weak input signal light is incident on the optical amplification section 25 that is excited by the excitation light, it is amplified and the input signal light is output from the optical amplification section 25 together with the excitation light. Is the excitation light blocking filter 26
The necessary amplified input signal light (including a slight amount of ASE noise) that has been removed by the above is re-input to the optical fiber 13 on the output side through the optical coupler 27 and travels to the receiving end station 14.

As the optical amplification section 25, a rare earth-doped fiber is widely used at present, and for example, erbium (E
r) It is common to use doped fibers. The operation is well known, and when excitation light energy is externally applied to the erbium-doped fiber f (for example, from the excitation light drive unit 24 including a laser diode (LD)), the electrons surrounding the erbium ion are high. It is excited to the energy level. When the input signal light is incident on this, so-called stimulated emission occurs, and the weak input signal light is exponentially amplified. Although FIG. 25 shows an example in which the pumping light is injected from the input end of the erbium-doped fiber f (forward pumping), the pumping light is injected from the output end of the erbium-doped fiber f in the opposite direction to the input signal light. It does not matter (backward excitation). In the configuration of the present invention described later, the configuration in which the forward excitation is adopted is shown, but the configuration in which the backward excitation is adopted may of course be adopted.

The performance that is first required for the optical amplifier 11 shown in FIG. 25 is a desired gain or a desired output signal light level (output power). That is, it is important that each optical amplifier 11 have a desired gain or produce a desired output power to meet a predetermined design level diagram that varies along the long distance optical fiber 13. is there. Therefore, in the conventional optical amplifier 11 shown in FIG. 25, the input side optical coupler 21 and the output side optical coupler 27 described above, the photodiodes (PD) 28 and 29 respectively paired therewith, and the photodiodes 28 and 29 A control circuit 23 is provided, which uses each output of 29 as a control input and generates a control output to the laser diode (LD) 24 described above.

[0007]

The cause of the problem to be solved by the present invention is the mismatch between the gain peak wavelength λ _{ASE · P} and the signal light wavelength λ _S. This will be described below. FIG. 26 is a diagram showing spectra when the gain peak wavelength matches the signal light wavelength (A) and when the gain peak wavelength does not match (B). Here, the signal light wavelength λ _S is the wavelength of light forming an optical signal transmitted between the terminal stations. The subscript S of λ means Signal. On the other hand, the gain peak wavelength λ
_{ASE · P} is a wavelength at which the ASE spectrum has a peak. In addition, ASE with the suffix of λ means amplified spontaneous emission (Amplified Spontaneous Emission), and P means peak. Generally, the gain of the optical amplifier (Gai
n) has wavelength dependence, and speaking of the gain peak wavelength,
Wavelength at which the gain of this optical amplifier reaches its peak λ _{G · P}
Means However, for example, if erbium-doped fiber optical amplifiers are cascaded in multiple stages (cascaded repeater
ters), and its gain peak wavelength λ _{G · P} is the wavelength λ at which the ASE spectrum becomes a peak each time the number of stages is increased.
It is known to approach _{ASE / P} infinitely. this
We call it "auto-filtering". [References PK Rung
e and EK Stafford, "AT & T optical amplifier system
s, "Sboptic '93, S.3,3.5, pp.72-77,1993]. Therefore, here, the gain peak wavelength λ _{G · P} is expressed as the wavelength λ _{AS E · P at} which the ASE spectrum has a peak. To

The optical amplifier 11 itself generates noise, albeit slightly, which is amplified and becomes spontaneous emission light ASE. Since a large number of optical amplifiers are used in the ultra-long-distance transmission line, the ASE generated in each optical amplifier accumulates to form the ASE spectrum shown in FIG. This ASE spectrum has a mountain shape as shown in the figure, and the wavelength corresponding to the apex is λ _{ASE · P.} In other words, the amplification efficiency of the signal light in this λ _{ASE · P} is the highest. Therefore, when the signal light wavelength λ _S and the gain peak wavelength λ _{ASE · P} coincide with each other, the transmission efficiency of the ultra-long-distance optical communication system becomes maximum and the SNR becomes the best, so that the transmission quality is necessarily high. Will be secured. FIG. 26A shows the ideal state (λ _S = λ _{ASE · P} ), and the noise power with respect to the transmission optical power P _ST is minimum, that is, SNR.
Is the maximum. The subscript T means tuning.

However, in reality, the above-mentioned ideal state does not occur, and normally λ _S ≠ λ _{ASE · P.} Then, the transmitted optical power becomes P _SD shown in the graph of (B),
The SNR is lowered (subscript D means detuning). Therefore, most of the signal light spectrum is buried in the ASE spectrum, and the SNR is currently deteriorated.

The characteristic shown in FIG. 26B is shown in FIG.
As a measure for improving the characteristics shown in (1), a method of matching the signal light wavelength λ _S with the gain peak wavelength λ _{ASE · P} can be considered first. However, this method is not easy. This is because normally λ _S needs to be set in the vicinity of the zero-dispersion wavelength of the optical fiber that is the transmission line, so the setting range is limited. As a result, as the transmission distance becomes longer and the transmission speed becomes higher, the problems of the chromatic dispersion of the optical fiber 13 and the nonlinear optical effect become more remarkable, and the range in which λ _S can be changed becomes extremely narrow. . At present, a huge distance of optical fiber is already installed all over the world. Each of them has its own zero dispersion wavelength, which cannot be changed later. Therefore, when determining the wavelength of the signal light passing through each of the optical fibers, it must be set near the zero dispersion wavelength determined for each of the optical fibers.
Then, the gain peak wavelength must be matched with the signal light wavelength. Also,
The degree of freedom (or range) for matching the gain peak wavelength and the signal light wavelength can be expanded.

After all, in order to improve the characteristic shown in FIG. 26B to the characteristic shown in FIG.
It is more advantageous to adopt the method of matching _{ASE · P} with the signal light wavelength λ _S. However, no specific proposal has yet been made to realize this method. Referring again to the conventional optical amplifier 11 shown in FIG. 25, the control circuit 23 shown here detects the level of the input signal light by the optical coupler 21 and the photodiode 28, and detects the level of the output signal light as an optical signal. It is detected by the coupler 27 and the photodiode 29 so that the ratio of these detection levels becomes constant, that is, the optical amplifier 1
The laser diode 24 is only driven so that the gain of the entire unit 1 becomes a desired constant value, and the intensity of the pumping light is merely adjusted. That is, in the optical amplifier 11 having the conventional circuit configuration, a control circuit for matching λ _{ASE · P} with λ _S cannot be found.

Λ_{ASE ・ P}Λ_SIs to match
Focusing on experimental facts, it can be achieved quite easily.
Wear. This will be described next. Fig. 27 shows rare earth dope
It is a graph showing the relationship between the gain of the Iba and the gain peak wavelength.
It The horizontal axis of this graph is the rare earth-doped fiber, for example,
Shows the gain G (gain) of rubium (Er) -doped fiber
The vertical axis shows the gain peak wavelength λ under each gain. _{ASE ・ P}
Indicates. Origin G_OAnd a certain gain peak wavelength λ_{ASE / PO}To
To take.

The graph shows that the gain peak wavelength λ _{ASE · P} changes when the gain of the rare earth-doped fiber (corresponding to the optical amplifying section 25 in FIG. 25) is changed. Specifically, it can be seen that there is a monotonically decreasing relationship in which when G is gradually increased from G _O , λ _{ASE · P} is also gradually decreased from λ _{ASE · PO} . The figure shows a linear decrease.

Then, it is understood that the gain G may be adjusted in order to match λ _{ASE · P} with λ _S. Therefore, it is understood that the strength of the excitation light (see FIG. 25) should be adjusted in order to make λ _{ASE · P} coincide with λ _S.
Note that the horizontal axis of FIG. 27 may be pumping light power. In this case, λ _{ASE · P} decreases as the pumping light power increases.

Referring again to FIG. 25, the control circuit 23 shown in FIG.
It operates so as to maintain the desired constant gain required for the. That is, the circuit 23 adjusts the intensity of the pumping light so that the gain G is not changed. However, on the other hand, in FIG.
As described above, the gain control circuit is required to adjust the intensity of the pumping light so that λ _{ASE · P} coincides with λ _S , based on the graph.

Here, there is a first requirement to match λ _{ASE · P} with λ _S and a second requirement to maintain the gain of the optical amplifier (or the output power of the optical amplification section 25) at a desired constant value. Two requests exist at the same time. Moreover, there is no connection between these two requirements. After all,
In a conventional optical amplifier, while maintaining its gain or output power at a desired constant value, the gain peak wavelength λ
There was a problem that _{ASE / P} could not be matched with the signal light wavelength λ _S.

Therefore, it is an object of the present invention to provide an optical amplifier capable of maintaining a gain or output power at a desired constant value and at the same time matching a gain peak wavelength with a signal light wavelength. is there.

[0018]

FIG. 1 is a diagram showing the principle configuration of an optical amplifier according to the present invention. In this figure, an optical amplifier control means 31 and a variable optical attenuating means 32 particularly form the main part of the present invention. It should be noted that the same reference numerals or symbols are given to the same components throughout the drawings.

First, the optical amplifier 11 which is the premise of the present invention is
It is an optical amplifier in an optical amplification multistage repeater transmission system in which a plurality of optical amplifiers are cascade-connected between transmitting and receiving terminal stations along an optical fiber. As a first example thereof, an output signal light obtained by combining the input signal light input with the pumping light by amplifying the input signal light, each of which has an optical amplification section 25 made of a rare earth-doped fiber (f in FIG. 2). Is an optical amplifier having a function of outputting The optical amplifier 11 incorporates the above-mentioned two means 31 and 32. That is, the gain peak wavelength λ _{ASE · P} in the optical amplification multistage repeater transmission system is set to the signal light wavelength λ of the input and output signal lights.
The optical amplifying section control means 31 for matching with _S and the level of the signal light power from the optical amplifying section 25 varied by the optical amplifying section control means 31 are adjusted so that the output signal light is set to a predetermined signal light power level. The optical attenuator 32 is newly provided in the optical amplifier 11.

As a concrete configuration of the optical amplifying section 25, a description will be given below by taking an ordinary rare earth-doped fiber as an example. However, as the optical amplifying section 25, other semiconductor optical amplifying elements are used. You can also do it. An example of using this semiconductor optical amplifier will be described later.

[0021]

As described with reference to FIG. 27, the gain of the optical amplifying section 25 having the rare earth-doped fiber f (optical amplifier 1
The gain peak wavelength λ _{ASE · P} can be changed by changing the gain peak wavelength λ _{ASE · P.} Therefore, in the optical amplifier control means 31, the pumping light controller 23 controls the pumping light driver 24 (see FIG. 2), and pumping light of an appropriate level is incident on the gain peak wavelength λ _{ASE · P.} It can be matched with the signal light wavelength λ _S.

On the other hand, by inputting the appropriate level of pumping light into the rare earth-doped fiber f, the optical amplifier 2
5 will have a gain dependent on that level of pump light. Then, the input signal light amplified with this gain is transmitted as the output signal light, and the signal light power level of the output signal light at this time becomes the desired level required for the optical amplifier. The likelihood of a match is extremely low. This is because the gain (G) set for the optical amplification section 25 is a value determined only for the purpose of matching λ _{ASE · P} with λ _S , and the gain (G) is set to a desired value. This is because no consideration is given to obtaining an output signal light having a signal light power level.

[0023] Therefore the desired (output power level of the optical amplifier 11) (P _O) high-level gain range of the signal light power is obtained in the area than the signal light power level, lambda
Select one gain where _{ASE P} matches λ _S. This satisfies the first requirement that λ _{ASE · P} coincides with λ _S. However, at this time, the signal light power level of the optical amplifier 25 is higher than the desired signal light power level (P _O ) and has not reached P _O.

[0024] Therefore, while it is satisfied the first request, i.e. the power of the excitation light remains on to thereby lower the output power level of the optical amplifier 11 to P _O that. The variable light attenuation means 32 plays this role. Thus, the gain or output power (signal light power) of the optical amplifier 11
The second requirement to maintain the level at the desired gain or power level will also be met along with the first requirement.

Thus, the gain or output power is
Λ while maintaining a constant value_{ASE ・ P}Λ _STo match
You can

[0026]

FIG. 2 is a diagram showing a specific example (A) of the configuration of FIG. 1 and a level diagram (B) inside thereof. First, referring to (A) of the figure, the pumping light control means 31, the variable light attenuation means 32, and the existing optical amplification section 25 located in the middle between them are provided at the positions shown in the figure. Has been. The components 22, 24, 26, 27 and 29 are as shown in FIG. As other existing components, the first optical isolator 15 provided at the input stage for receiving the input signal light, and the second optical isolator provided between the output stage of the optical amplifier 25 and the variable optical attenuator 32. And an isolator 16. In this case, the excitation light blocking filter 2
6 is inserted between the second optical isolator 16 and the variable optical attenuator 32. The isolators 15 and 16 allow the signal light to travel in only one direction, and cut off unnecessary reflected light here to stabilize the circuit operation.

In FIG. 2, the variable light attenuation means 32 is used.
1 shows an example in which an optical attenuator (ATT) and a control circuit are configured. Considering the column (B) of FIG. 2 together with the column (A), the input signal light (power P _i ) input from the left side of the figure passes through the optical amplifier 11 and is output from the right side of the figure. A change in the power level (dBm) until the signal light (power P _O ) is output is shown as a level diagram. In this case, P _O is a desired power level that the optical amplifier 11 should output by design. As a design requirement, the gain (G) of the optical amplifier 11 may be designated to a desired value in addition to the case where the power level P _O is designated to a desired value. The gain G at this time is represented by G = P _O / P _i .

The level of the input signal light input at the power level P _i is gradually attenuated to the minimum level P _fi at the input end of the rare earth-doped fiber f. The input signal light amplified by the pumping light in the fiber f is output from its output end at the maximum level of P _fO . in this case,
Gain fiber f (g) is represented by g = P _fO / P _fi. This gain (g) is the largest factor that shifts the gain peak wavelength λ _{ASE · P} , as described in FIG. Therefore, as described above, the intensity of the pumping light is adjusted by the optical amplifying unit control means 31, thereby adjusting the gain g so that λ _{ASE · P} and the signal light wavelength λ _S match.
In this case, the power level P at the output end of the fiber f
_{fO needs} to be greater than the desired output power level P _O.

The input signal light output from the fiber f at a level exceeding P _O is attenuated again, and the variable optical attenuator (A
It is further attenuated by TT) and re-enters the optical fiber (13) as an output signal light having a desired power level P _O. At this time, the level P _fO at the output end of the fiber f
Must be exactly brought to P _O. The variable light attenuating means 32 controls this. For example, the control circuit monitors a part of the output signal light by the optical coupler 27 and the photodiode 29, and performs feedback control such that the feedback control is held at P _O while comparing a predetermined reference level V _ref with a variable optical attenuator (ATT). Do against.

FIG. 3 is a diagram showing a first embodiment of the present invention, FIG.
FIG. 7 is a diagram showing a second embodiment of the present invention. Under the first embodiment (FIG. 3), the optical amplification section control means 31 controls the gain peak wavelength λ in the optical amplification multistage repeater transmission system (FIG. 24).
The deviation between the _{ASE · P} and the signal light wavelength λ _S is monitored, control information from the receiving terminal station 14 (FIG. 24) indicating the deviation is received, and the excitation light is adjusted so as to minimize the deviation. It is to adjust the power.

On the other hand, under the second embodiment (FIG. 4), the variable optical attenuating means 32 shifts the deviation between the gain peak wavelength and the signal light wavelength in the optical amplification multistage repeater transmission system (FIG. 24). The receiving terminal station 14 (see FIG.
4) The control information from 4) is received, the level of the signal light power from the optical amplifier 25 is changed, and the change in the power of the output signal light according to this change is fed back to the optical amplifier controller 31. To do.

As described above, in the optical amplification multistage repeater transmission system (FIG. 24), each optical amplifier 11 (a) how much output power level (P _O ) should be sent out, or (b) ) What kind of gain G should be given is predetermined in the design of the transmission system. That is, each optical amplifier 1
1 operates in (a) constant output control mode or (b) constant gain control mode. Therefore, under the configuration of the first embodiment, the two cases of operating in the mode (a) or (b), and under the configuration of the second embodiment, the mode (a) or ( There are two cases of operating in b), and a total of four cases are assumed. Hereinafter, configuration examples of these four cases (FIGS. 5 to 8) will be individually described with reference to the drawings. In each figure, for the sake of simplification, the isolator 15,
16 is omitted.

FIG. 5 shows a first embodiment under the first embodiment of the present invention.
FIG. 6 shows a second embodiment under the first embodiment of the present invention, and FIG. 7 shows a first embodiment under the second embodiment of the present invention. FIG. 8 and FIG. 8 are views showing a second mode under the second embodiment of the present invention. The first mode is the constant output control mode (a) described above, and the second mode is the constant gain control mode (b) described above.

First, in FIG. 5, the variable light attenuation means 32 is used.
Is a signal light output control circuit 41 for monitoring the power of the output signal light and controlling it so as to maintain the power at a desired constant value.
Equipped with. The optical amplification unit control means 31 changes the intensity of the excitation light depending only on the command (control information) from the terminal station 14. Therefore, it is unknown what kind of power level signal light is output from the rare earth-doped fiber f. However, no matter what the power level is, the variable optical attenuator 32 and the signal light output control circuit 41 are finally connected.
The output power level of the output signal light is forced to P
Maintained at _O.

The optical amplification section control means 31 in this figure includes means for controlling the pumping light drive section 24 in accordance with the control information from the terminal station 14. If the control from the terminal station 14 is given by a radio signal passing through a communication path different from that of the optical fiber 13, it becomes a means including at least a radio receiver. If the control information from the terminal station 14 is given as an optical signal transmitted by being multiplexed with the main signal in the optical fiber 13, it becomes a means including an optical / electrical converter (O / E).

Further, the signal light output control circuit 41 is
Circuit for converting output light from the ion 29 into an electric signal
(O / E) and the output of that circuit (O / E)
Force and reference level V_refWith the second comparison input as
It is configured to include a generator. In FIG.
Is the variable optical attenuator 32, which is the power of the input signal light.
(P_i) And the power of the output signal light (P_O) For each
The ratio of both powers (P_O/ P_i= G) becomes constant
The gain G of the optical amplifier 11 is controlled by controlling
A gain control circuit 42 for maintaining a desired constant value is provided. Light increase
The width section control means 31 issues a command (control information) from the terminal station 14.
The intensity of the excitation light is changed only depending on. But
What is the power level from the rare earth-doped fiber f?
It is unknown whether the bell signal light will be output. But which
Even though the power level is
By the attenuation means 32 and the gain control circuit 42, the output signal light
Output power level P _OInput signal power level
Le P_i, The gain G (= P_O/ P_i) Is compulsory
Is maintained at a desired constant value. This gain control circuit 42
Is as simple as the signal light output control circuit 41.
It is not possible to configure only the
Introduces a typical configuration.

The optical amplifier control means 31 is as described with reference to FIG. In FIG. 7, the optical amplification unit control means 31 includes a signal light output control circuit 41 that monitors the power P _O of the output signal light and controls the power so as to maintain the power at a desired constant value. The variable optical attenuator 32 receives the control information from the terminal station 14 which monitors the deviation between the gain peak wavelength λ _{ASE · P} and the signal light wavelength λ _S and indicates the deviation amount. Note that this control information may be transmitted by a wireless signal or an optical signal as already described in FIG.

The control information from the terminal station 14 allows
The attenuation amount of the variable light attenuator 32 changes. Then the light
Obtained through coupler 27 and photodiode 29
The level of the output signal light also changes. This change is signal light output
Reference level V of control circuit 41 _refAs a change to
It is fed back to the optical amplifier control means 31 and changes the intensity of the pumping light.
Let In this feedback, the output signal light level is V_refOne
It is done until it hits. At the time of this match, λ
_{ASE ・ P}And λ_SThe terminal station 14 detects that
If issued, the terminal station 14 again sends control information. same
The above procedure is repeated until the power level of the output signal light is finally
Bell is P_OAnd λ at the terminal station 14 _{ASE ・ P}But
λ_SIf it is confirmed that the above feedback control
finish.

The configuration of the optical amplification section control means 31 in this figure is almost the same as the configuration of the optical amplification section control means 31 shown in FIGS. 5 and 6. This is because the control input to the means 31 is simply changed. In FIG. 8, the optical amplification section control means 31 monitors the power P _i of the input signal light and the power P _O of the output signal light, respectively, and monitors the ratio of both powers (= P _o).
The gain control circuit 42 maintains the gain (= G) of the optical amplifier 11 at a desired constant value by controlling / P _i ) to be constant.

According to the control information from the terminal station 14, the variable light
The amount of attenuation of the attenuation means 32 changes. Then the light cap
Output obtained through laser 27 and photodiode 29
Signal light level P_OAlso changes. This change is due to gain control
The light is fed back to the optical amplification unit control means 31 via the path 42 and excited.
Change the intensity of light. This feedback is performed by the optical coupler 21 and
And the input signal light obtained through the photodiode 28.
Bell P_iAnd the above P_OAnd the gain G (= P_O
/ P_i) Until the desired constant value is reached. This desired
When a constant value of λ is obtained, λ_{ASE ・ P}And λ_SWhen
If the terminal station 14 detects that the
And the terminal station 14 sends control information. Repeat the same
And finally output signal light power P_OAnd the input signal light
Power P _iAnd the ratio (gain G) becomes a desired constant value,
At the terminal station 14_{ASE ・ P}Is λ _SIt is certain that
If so, the above feedback control ends.

The configuration of the optical amplification section control means 31 in this figure is almost the same as the configuration of the optical amplification section control means 31 shown in FIGS. 5 and 6. This is because the control input to the means 31 is simply changed. The configuration of the gain control circuit 42 is almost the same as the configuration of the gain control circuit 42 shown in FIG. 6, and one configuration example will be introduced next.

FIG. 9 is a diagram showing an example of the configuration of the gain control circuit 42. The gain control circuit 42 shown in the figure includes a photodiode 29 that receives the signal light from the output side optical coupler 27,
The input stage is provided with a photodiode 28 (21, 27, 28, 29, see FIGS. 6 and 8) for receiving the signal light from the input side optical coupler 21. On the other hand, the output control voltage V _CO from the output stage is applied to the variable optical attenuator 32 (in the case of FIG. 6) or the optical amplifier control unit 31 (in the case of FIG. 8).

Input control voltage V _C1 from the photodiodes 28 and 29 obtained via the load resistors R _L1 and R _L2
And V _C2 are input to LOG amplifiers 51 and 52, respectively, and the amplified outputs from these amplifiers 51 and 52 are applied to the inverting and non-inverting inputs of a differential amplifier (op amp) 53. Output V _in the amplifier 53 is input to the comparator 54, its output is the output control voltage V of the aforementioned
It becomes _CO .

Here, the amplification factor of each amplifier is shown in the figure, and A
1, A2 and A3, the above V _in is represented by V _in = A1 (LogP _O −LogP _i ) = A1 × LogP _O / P _i (for P _O and P _i, refer to FIG. 2). Here P _O
/ P _i is the gain G, and V _in is proportional to the logarithmic value of the gain G. On the other hand, the reference level V _ref that produces a predetermined desired constant gain is set, and the difference between V _in and V _ref is _calculated by the comparator 54. Here, the output control voltage V _CO is represented by V _CO = A3 × (V _in −V _ref ).

Finally, the variable light attenuation means 32 will be considered. The variable light attenuating means includes, for example, a mechanical control type using a plurality of polarizing elements or a combination of right-angle prisms, but the variable light attenuating means 32 in the present invention is an electrically controlled type that can be easily controlled remotely. Must be used, and this example will be described below. Note that this remote control may be performed collectively for a plurality of optical repeater amplifiers in the ultra-long-distance optical communication system, or may be performed individually.

FIG. 10 is a diagram showing a first configuration example of the variable light attenuation means. In the first configuration example, the variable light attenuation means 32
Is realized by the variable optical attenuator 61 configured as an electrically controllable Faraday rotator. The variable optical attenuator 61 includes a polarizer 71 and a coil 7 that forms a magnetic field H.
2, an yttrium iron garnet crystal YIG placed inside the coil 72, and an analyzer 73. First, the input light (left side in FIG. 10) that has been converted into parallel light by a lens (not shown) is divided into polarized light parallel to the paper surface and polarized light perpendicular to the paper surface by a polarizer (for example, a rutile polarization separation plate) 71. YI in coil 72
It is incident on G. This constitutes a so-called Faraday rotator, and the polarized wave rotates in this rotator according to the strength of the magnetic field H. The strength of this magnetic field H changes according to the control current I _C.

Further, the analyzer (for example, rutile) 73 adjusts so that the output light is irradiated to the central position of the optical coupler 27 in a certain magnetic field H. Control current I _C to approach this magnetic field H or to move away from this magnetic field H
If the control is performed, the irradiation efficiency of the output light to the optical coupler 27 changes, and a variable optical attenuator is realized. 11 is the same as FIG.
It is a diagram showing a practical realization method of the variable light attenuation means of
It is a polarization-independent optical isolator that is widely used at present and has an input / output optical fiber (corresponding to the optical fiber 13 of the present invention). The AR coat and the lens collimate the incident light and apply it to a polarizing element (corresponding to the polarizer 71 in FIG. 10). Further, the light enters the polarizing element (corresponding to the analyzer 73 in FIG. 10) via YIG, enters the optical fiber through the lens and the AR coat. FIG. 12 is a diagram showing the principle of the above-mentioned polarization-independent optical isolator which is widely used at present. The entire amount of light in the forward direction (a) enters the lens, but in the reverse direction (b). Light is incident on the lens only partially, thus realizing an optical isolator.

FIG. 13 is a sectional view of a known optical isolator (see Electronics Essentials No. 11, published in Japan Industrial Technology Center, "Points of design and application of optical communication micro-optical system"). In this figure, the polarizer 7
The magnetic field (H) is applied to the YIG sandwiched between 1 and the analyzer 73 by a permanent magnet (for example, samarium.
Cobalt permanent magnet) 72 ', on which the Faraday rotator is formed.

FIG. 14 is a graph showing the magnetic field dependence of the Faraday rotation angle (see Electronics Essentials No. 11, "Points of design and application of optical communication micro-optical system", published by Japan Industrial Technology Center). In this graph, focusing on the characteristic curve of forward loss, the magnetic field H
By changing the, the transmission loss continuously changes by 25 dB. Using this forward loss characteristic curve,
Variable optical attenuator 32, which is one of the main constituent elements of the present invention.
Is realized.

Therefore, instead of the permanent magnet 72 'shown in FIG. 13, a means capable of continuously and electrically changing the strength of the magnetic field is adopted. This means is the coil 72 shown in FIG. 11 and forms an electromagnet. FIG. 15 is a diagram showing a second configuration example of the variable light attenuating means. This second
In the configuration example, the variable optical attenuator 32 is realized by the variable optical attenuator 62 composed of lithium niobate (LiNbO ₃ ) having an electro-optical effect.

The output light from the filter 26 enters the Z-plate LiNbO ₃ through the input side fiber. This incident light is guided to a Ti diffusion optical waveguide formed by diffusing titanium (Ti) in the Z plate LiNbO ₃ and reaches the output side fiber. At this time, the optical waveguide forms a mode-coupling Y branch, and the input light incident from the input side fiber is split into two waves. However, after that, they are combined again. Two
When the voltage V is not applied to the traveling wave electrode, the signal light demultiplexed into the waves travels in the same phase as it is and is incident on the output side fiber at the same level as the level of the input light from the input side optical fiber. It

However, when the above voltage V is applied, 2
There is a difference in the optical path lengths of the first light and the second light split into the waves. Therefore, the phases of the first and second lights at the time of being incident on the output side fiber are shifted. If this phase difference is 180 °, the input light from the input side fiber is attenuated to almost zero. Therefore, the above voltage V
Is changed by the control information from the terminal station or the output (V _CO ) from the gain control circuit 42, a variable optical attenuator is realized.

The LiNbO ₃ block is merely a mechanical holding member. Ruby beads are fiber and Z
It is a bonding member with the plate LiNbO ₃ . FIG. 16 is a diagram showing a third configuration example of the variable light attenuating means. In the third configuration example, the variable optical attenuator 32 is composed of a semiconductor modulator 63 having a well-known Franz-Keldysh effect.

The characteristics that the semiconductor modulator 63 should have are shown in FIG. 17, where the horizontal axis represents the control voltage V _C and the vertical axis represents the transmittance. Thus, by varying V _C , a variable optical attenuator using a semiconductor modulator is realized. The control voltage V _C is determined according to the control information from the terminal station or the output (V _CO ) from the gain control circuit 42. by the way,
Variable optical attenuator 62 composed of lithium niobate,
The semiconductor modulator 63 is required to have no polarization dependence, but it is difficult to realize at present, and it is difficult to inexpensively and easily manufacture a semiconductor modulator having no polarization dependence with a single device. Currently difficult.

Therefore, by arranging two devices 81 and 82 having the same structure as each other in different directions as shown in FIG. 18, although the individual devices 81 and 82 have polarization dependence, these two devices 81 and 82 have a polarization dependency. It is possible to equivalently prevent the polarization dependence from appearing by combining.
FIG. 19 is a perspective view showing a specific example of the device shown in FIG. In the figure, reference numeral 91 is GaInAsP, and the input light (from the filter 26) incident on the electrode 94 is
The output light (to the optical coupler 27) passes through with a transmittance determined by the control voltage V _C applied between and 99. In addition, 92 is p-InP, 93 is p-GaInAsP, 95
Is Polyimide, 96 is SiO ₂ , 97 is Si-
InP and 98 are n-InP.

As described at the end of the description of FIG. 1 described above, the optical amplifying section 25 can be realized by using a semiconductor optical amplifying element in addition to the case of using the above-mentioned general rare earth-doped fiber. . At present, it is natural to use rare earth-doped fibers, but they are quite expensive and have to be cheaper in order to be widely used in the vast number of already installed optical fibers. The most effective method for satisfying such requirements is the adoption of the semiconductor optical amplification element.

This semiconductor optical amplification element can be used as the optical amplification section 25 also in this semiconductor optical amplification element.
This is because the present inventor has noticed that the same characteristics as those shown in the graph of No. 7 are exhibited. FIG. 20 is a characteristic diagram of a known semiconductor optical amplifier (see Ohmsha, "Optical amplifier and its application", supervised by Ishio, Nakagawa et al., P. 50). Observing the characteristics of this figure, it can be seen that the peak wavelength of the signal gain shifts in the direction of the arrow in the figure as the injection current (60 mA, 70 mA ...) Increases. That is, it shifts to the shorter wavelength side.

FIG. 21 is a graph showing the relationship between the gain peak wavelength λ _P and the gain G. The tendency of this characteristic is monotonically decreasing like the tendency of the characteristic shown in FIG. However, the figure shows a linear decrease. The gain G of the semiconductor optical amplifier is
Depends on the amount of current injection supplied to the semiconductor optical amplification element. Since the rare earth-doped fiber optical amplifier and the semiconductor optical amplifying element described above can be used as the optical amplifying section 25 in the same manner, the configurations of the embodiments (FIGS. 3 to 8) disclosed so far can be applied to the semiconductor optical amplifying element as they are. Applicable. FIG. 22 illustrates one of them.

FIG. 22 is a diagram showing an example in which the same structure as in FIG. 8 is realized by using a semiconductor optical amplifier element. In the figure, a semiconductor optical amplifier element that replaces the rare earth-doped fiber f is designated by reference numeral 101. Along with this, the optical amplifier control means 31
Serves as the current injection unit 102. FIG. 23 is a diagram showing a configuration example of the current injection unit and the gain control circuit in FIG. The gain control circuit 42 is the same as the circuit shown in FIG. 9 except that the inverting and non-inverting inputs of the comparator 54 have been swapped.

The output control voltage Vco is applied to the current injection section 102 which constitutes the optical amplification section control means 31. The current injection unit 102 is a voltage buffer (BUF) that supplies a large current.
It comprises 111 and a protection resistor 112. In addition, in the configuration of FIG. 5 and FIG. 6, the current injection unit 102 in the case of adopting the semiconductor optical amplifier element instead of the rare earth-doped fiber f.
Means that the control information from the terminal station is applied instead of the output control voltage Vco.

The semiconductor optical amplifier element is a semiconductor laser amplifier (TWSLLA: Traveling Wave Semiconductor Lase).
It is also called r Amplifier), and there are "traveling wave type" and "reflective type" in its operation form. The semiconductor optical amplifier device of the present invention is preferably of a traveling wave type (the rare earth-doped fiber f is also a traveling wave type). This is because it has excellent operational stability.

[0062]

As described above, according to the present invention, while maintaining the gain or output power at a desired constant value,
An optical amplifier that can match the gain peak wavelength with the signal light wavelength is provided, the SNR is dramatically improved, and an ultra long-distance optical communication system having high transmission quality can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle configuration of an optical amplifier according to the present invention.

FIG. 2 is a diagram showing a specific example (A) of the configuration of FIG. 1 and a level diagram (B) therein.

FIG. 3 is a diagram showing a first embodiment of the present invention.

FIG. 4 is a diagram showing a second embodiment of the present invention.

FIG. 5 is a diagram showing a first mode under the first embodiment of the present invention.

FIG. 6 is a diagram showing a second mode under the first embodiment of the present invention.

FIG. 7 is a diagram showing a first mode under the second embodiment of the present invention.

FIG. 8 is a diagram showing a second mode under the second embodiment of the present invention.

FIG. 9 is a diagram showing a configuration example of a gain control circuit 42.

FIG. 10 is a diagram showing a first configuration example of variable light attenuating means.

FIG. 11 is a diagram showing a variable optical attenuator using an optical isolator.

FIG. 12 is a diagram showing the principle of an optical isolator.

FIG. 13 is a sectional view of a known optical isolator.

FIG. 14 is a diagram showing the magnetic field dependence of the Faraday rotation angle.

FIG. 15 is a diagram showing a second configuration example of the variable light attenuating means.

FIG. 16 is a diagram showing a third configuration example of the variable light attenuating means.

FIG. 17 is a diagram showing characteristics that the semiconductor modulator of FIG. 16 should have.

FIG. 18 is a diagram showing a practical configuration example of a semiconductor modulator.

19 is a perspective view showing a specific example of the device shown in FIG.

FIG. 20 is a characteristic diagram of a known semiconductor optical amplifier device.

FIG. 21 is a graph showing the relationship between the gain peak wavelength λ _P entrance and the gain G.

22 is a diagram showing an example in which the configuration of FIG. 8 is realized by a semiconductor optical amplification element.

23 is a diagram showing a configuration example of a current injection unit and a gain control circuit in FIG.

FIG. 24 is a diagram showing a general configuration of an optical amplification multistage repeater transmission system.

FIG. 25 is a diagram showing a conventional example of an optical amplifier.

FIG. 26 is a diagram showing spectra when the gain peak wavelength matches the signal light wavelength (A) and when the gain peak wavelength does not match (B).

FIG. 27 is a graph showing the relationship between the gain of rare-earth-doped fibers and the gain peak wavelength.

[Explanation of symbols]

 10 ... Optical amplification multi-stage repeater transmission system 11 ... Optical amplifier 12 ... Optical amplification repeater 13 ... Optical fiber 14 ... Terminal station 15 ... First isolator 16 ... Second isolator 21, 27 ... Optical coupler 22 ... Multiplexer 23 ... Excitation Optical control unit 24 ... Excitation light driving unit 25 ... Optical amplification unit 26 ... Excitation light blocking filter 31 ... Optical amplification unit control unit 32 ... Variable optical attenuation unit 41 ... Signal light output control circuit 42 ... Gain control circuit 61, 62 ... Variable Optical attenuator 63 ... Semiconductor modulator 101 ... Semiconductor optical amplifier element 102 ... Current injection unit 111 ... Voltage buffer 112 ... Protective resistance

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G02F 1/35 501 H04B 10/17 10/16

Claims (13)

[Claims] 1. An optical amplifier (11) in an optical amplification multistage repeater transmission system comprising a plurality of optical amplifiers connected in cascade along an optical fiber (13) between transmitting and receiving terminal stations (14). An optical amplifier for outputting an output signal light, which is obtained by amplifying an input signal light, which has an optical amplifying section (25), each of which comprises a rare earth-doped fiber or a semiconductor optical amplifying element, and a gain peak in the optical amplification multistage repeater transmission system. An optical amplification section control means (31) for matching the wavelength with the signal light wavelength of the input and output signal light, and adjusting the level of the signal light power from the optical amplification section varied by the optical amplification section control means. Then, the variable optical attenuating means (32) for setting the output signal light to a predetermined signal light power level
And an optical amplifier. 2. The optical amplification section control means (31) monitors the deviation between the gain peak wavelength and the signal light wavelength in the optical amplification multistage repeater transmission system and indicates the deviation amount. The control information from (14) is received, and the power of the pumping light applied to the optical amplification section (25) made of the rare earth-doped fiber is adjusted so as to minimize the deviation amount. Optical amplifier. 3. The optical amplification section control means (31) monitors the deviation between the gain peak wavelength and the signal light wavelength in the optical amplification multistage repeater transmission system and indicates the deviation amount. The light according to claim 1, wherein the control information is received from (14), and the amount of current injection supplied to the optical amplification section (25) composed of the semiconductor optical amplification device is adjusted so as to minimize the amount of deviation. amplifier. 4. The variable optical attenuator (32) monitors the deviation between the gain peak wavelength and the signal light wavelength in the optical amplification multistage repeater transmission system, and indicates the deviation amount by the receiving terminal station (14). ) Is received to change the level of the signal light power from the optical amplifying section (25), and the change in the power of the output signal light according to this change is controlled by the optical amplifying section control means (31). The optical amplifier according to claim 1, wherein the optical amplifier is configured to be returned to. 5. The variable optical attenuator (32) includes a signal light output control circuit (41) for monitoring the power of the output signal light and controlling the power so as to maintain the power at a desired constant value. 2. The optical amplifier according to 2 or 3. 6. The variable optical attenuator (32) monitors the power of the input signal light and the power of the output signal light, respectively, and controls so that the ratio of both powers becomes constant, thereby controlling the optical amplifier. The gain control circuit (42) for maintaining the gain of (11) at a desired constant value.
The optical amplifier according to. 7. The optical signal output control circuit (41), wherein the optical amplification section control means (31) monitors the power of the output signal light and controls so as to maintain the power at a desired constant value.
The optical amplifier according to claim 4, further comprising: 8. The optical amplification section control means (31) monitors the power of the input signal light and the power of the output signal light, respectively, and controls them so that the ratio of both powers becomes constant. An optical amplifier according to claim 4, comprising a gain control circuit (42) for maintaining the gain of the amplifier (11) at a desired constant value. 9. The optical amplifier according to claim 1, wherein the variable optical attenuating means (32) comprises a variable optical attenuator (61) configured as an electrically controllable Faraday rotator. 10. The variable light attenuating means (32) comprises lithium niobate (LiNb) having an electro-optical effect.
The optical amplifier according to claim 1, comprising a variable optical attenuator (62) composed of O ₃ ). 11. The optical amplifier according to claim 1, wherein the variable optical attenuation means (32) is composed of a semiconductor modulator (63) having a Franz-Keldysh effect and having no polarization dependence. 12. A first optical isolator (15) provided in an input stage for receiving the input signal light, and an output stage of the optical amplification section (25) and the variable optical attenuating means (32). Optical amplifier according to claim 1, comprising a second optical isolator (16). 13. The optical amplifier according to claim 11, further comprising a pumping light blocking filter (26) inserted between the second optical isolator (16) and the variable optical attenuator (32).