JP2001060917A - Wavelength multiplex light transmitter and light amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration of transmission quality while reducing a manual load by preventing an inter-channel cross talk from deteriorating even if an output light wavelength of a light transmitter fluctuates. SOLUTION: A part of a light signal transmitted from a light transmitter is branched by a directional coupler 1 just before it is introduced into a light amplification part 2 and it is divided into two systems by a directional coupler 3. The light signal of one system is introduced to a Pin photodiode 4 as it is and is photoelectrically converted. The light signal of the other system is introduced into the Pin photodiode 6 through a light filter 5 and is photoelectrically converted. The photoelectrically converted outputs of the Pin photodiodes 4 and 6 are given to a differential amplifier 8 and the amplification output is compared with threshold voltage that a reference voltage source 11 gives by a comparator 10. When the output of the differential amplifier 8 exceeds the threshold, the light amplification part 2 is turned off by an output cut circuit 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical transmitter and an optical amplifier used in a wavelength division multiplexing optical transmission system.

[0002]

2. Description of the Related Art A wavelength division multiplexing optical transmission system is a system for expanding transmission capacity by multiplexing optical signals having different wavelengths on a single optical fiber, thereby reducing the cost of laying optical fibers and the cost of repeaters. In addition, since large-capacity long-distance transmission is possible, it has been actively researched in recent years and is being put to practical use.

FIG. 12 shows a configuration of a wavelength division multiplexing optical transmission device used in a wavelength division multiplexing optical transmission system. In the figure, the optical signals λ1 to λn having different wavelengths transmitted from the optical transmitters 101 to 10n respectively correspond to the optical amplifier 201.
After being amplified to a desired power by .about.20n, it is wavelength-multiplexed by the wavelength multiplexer 300 and transmitted to a transmission line (not shown).

By the way, in the above configuration, the optical transmitter 101
It is necessary to pay attention to the wavelength shift of 10 to 10 n. This will be described with reference to FIG. In FIG. 13, for example, it is assumed that the output light wavelength of the optical transmitter 10 n gradually increases from the stable point λn due to aging or deterioration. If the output wavelength of another optical transmitter is located ahead of it, the output light of the shifted optical transmitter 10n interferes with the optical signal of another channel as shown in FIG.

The optical transmitters are often individually mounted on the housing in the form of a board, but the same occurs when each optical transmitter is replaced. For example, if the optical transmitter 10n is replaced, an optical signal of λ0 corresponding to room temperature is output immediately after startup after the replacement, but the output light wavelength shifts to λn as the chip gradually warms up. I will do it.
This is because the output light wavelength of the optical transmitter is generally controlled by adjusting the temperature of the laser-oscillating chip. If the wavelength crosses the wavelength output by the optical transmitter of the other channel during that time, the output light of the shifted optical transmitter 10n and the optical signal of the other channel interfere as shown in (B).

[0006] When optical signals interfere with each other, crosstalk between channels deteriorates, causing deterioration of transmission quality such as reception error. In order to avoid such deterioration of transmission quality, the wavelength shift is prevented beforehand by monitoring the output light wavelength of each optical transmitter periodically, or until the output light wavelength of the optical transmitter becomes stable. Countermeasures such as turning off the optical amplifier at the subsequent stage are taken. However, all of these methods have a large human load and are inconvenient in operation.

[0007]

As described above, in the conventional wavelength division multiplexing optical transmission apparatus, when the output light wavelength of the optical transmitter fluctuates, the optical signals of other wavelengths are affected, and the crosstalk between channels is affected. However, there is a problem that the transmission quality is deteriorated due to the deterioration of the data transmission, and the human load for preventing the problem is large.

[0008] The present invention has been made in view of the above circumstances,
An object of the present invention is to provide a wavelength division multiplexing optical transmitting apparatus and an optical amplifier that prevent crosstalk between channels from deteriorating even when the output light wavelength of an optical transmitter fluctuates, thereby reducing the human load and suppressing the deterioration of transmission quality. Is to provide.

[0009]

In order to achieve the above object, the present invention provides a plurality of optical transmitters to which transmission wavelengths different from each other are assigned, and a plurality of optical transmitters provided for each of the plurality of optical transmitters. A plurality of optical amplifiers for amplifying and outputting optical signals transmitted from the respective optical transmitters; and a wavelength multiplexer for wavelength-multiplexing the output light from the plurality of optical amplifiers and transmitting the multiplexed light to an optical transmission line. In the wavelength multiplexing optical transmitter,
Each of the plurality of optical amplifiers is provided in an optical waveguide extending from an input port to an output port, and is configured to amplify an optical signal transmitted from the optical transmitter to be connected to the optical waveguide via the input port. Amplifying means, a branching means for partially branching and extracting an optical signal passing through the optical waveguide, and comparing a wavelength of the branched light extracted by the branching means with a transmission wavelength assigned to the optical transmitter of the connection partner. And a wavelength shift detection unit that outputs an output cutoff signal when there is a shift in these wavelengths, and an output cutoff unit that turns off the optical signal output from the optical amplification unit when the output cutoff signal is given. It is provided with.

By taking such means, the amplified light transmitted from the optical amplifying means is partially branched and guided to the wavelength shift detecting means. In the wavelength shift detecting means, the wavelength of the branched amplified light is compared with the transmission wavelength assigned to the optical transmitter of the connection partner. Then, when there is a difference between these wavelengths, the output signal output means turns off the optical signal output from the optical amplification means.

Therefore, when the optical transmitter is transmitting an optical signal having a wavelength different from the originally assigned wavelength,
The optical signal is interrupted at the stage of the optical amplifier, and is not transmitted to the wavelength multiplexer and thereafter. As a result, even if a wavelength shift occurs in the optical transmitter, it does not adversely affect the optical signals of other channels in the optical transmission path, thereby avoiding deterioration of crosstalk between channels and suppressing deterioration of transmission quality. Becomes

When the wavelength shifts, the signal light is automatically cut off by the operation of the wavelength shift detecting means and the output cutting means. Accordingly, it is not necessary to monitor the wavelength manually, and it is possible to reduce a human load.

According to another aspect of the present invention, there is provided an excitation light source and an amplification optical fiber doped with a rare earth element, and the excitation light generated by the excitation light source is guided to the amplification optical fiber and passes through the amplification optical fiber. An optical amplifier for amplifying an optical signal, comprising pulse modulating means for performing pulse modulation on pumping light guided to the amplification fiber, and varying a duty ratio of a pulse of the pumping light, in the amplification optical fiber. A desired amplification gain is obtained by controlling the average pump light power.

In particular, the optical amplifier according to the present invention includes a feedback loop for variably controlling the duty ratio of the pump light in accordance with the level of the amplified light transmitted from the amplifying optical fiber so as to keep this level constant. It is prepared for.

By taking such measures, the amplification optical fiber is excited by pulsed excitation light.
Generally, by setting the frequency of the pulse to be preferably 100 times or more the lifetime of the rare earth ion doped into the amplification optical fiber, a gain equivalent to that of pumping by continuous light can be obtained.

When the amplification optical fiber is pumped by the pulsed pumping light, when the operating characteristics of the pumping light source temporarily fluctuate and the pumping light output level fluctuates, this fluctuation is absorbed by the pulse change. .

Therefore, even if the level of the pumping light source fluctuates instantaneously due to mode hopping or return light, the total power of the pumping light applied to the amplification optical fiber is constant. This makes it possible to operate the optical amplifier stably.

[0018]

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

(First Embodiment) FIG. 1 is a block diagram showing the configuration of an optical amplifier according to the present embodiment. This optical amplifier is used in place of the optical amplifiers 201 to 20n in the wavelength division multiplexing optical transmitter shown in FIG. In the following description, it is assumed that an optical amplifier is connected at the position indicated by reference numeral 201 in FIG. 12, and therefore, the wavelength of the optical signal to be handled is λ1. Other optical amplifiers have the same configuration as that of FIG.

In FIG. 1, an optical signal of wavelength λ1 transmitted from an optical transmitter (not shown) is amplified to a predetermined power by an optical amplifier 2 via a directional coupler 1, and then a wavelength multiplexer (not shown) Sent to The directional coupler 1 outputs the optical signal λ
1 is divided into two systems, one of which is guided to the optical amplification unit 2 and the other is guided to the directional coupler 3 at a ratio of, for example, 9: 1. In short, most of the optical signal is directly supplied to the optical amplifier 2, and a part of the optical signal is guided to the directional coupler 3.

Although not shown, the optical amplifying unit 2 includes, for example, Er.
The optical fiber amplifier includes a so-called optical fiber amplifier that performs optical amplification by guiding excitation light from an excitation light source (not shown) to an amplification fiber doped with a rare earth such as (erbium).

The directional coupler 3 further divides the branched optical signal into two at a ratio of, for example, 1: 1. It leads to the Pin photodiode 6.
The optical filter 5 selectively transmits an optical signal having the wavelength λ1, and its transmission band is set to be the same as the output light wavelength of the optical transmitter connected to the preceding stage.

The Pin photodiode 4 is for photoelectrically converting an optical signal through two directional couplers 1 and 3 in response to the optical signal. The obtained photocurrent is amplified to a predetermined intensity by a logarithmic amplifier 7. Thereafter, the signal is supplied to one input terminal of the differential amplifier 8. The Pin photodiode 6 photoelectrically converts an optical signal passing through the directional couplers 1 and 3 and the optical filter 5, and a photocurrent is amplified to a predetermined intensity by a logarithmic amplifier 9 and the other input terminal of the differential amplifier 8. Given to.

The output of the differential amplifier 8 is compared in a comparator 10 with a threshold voltage generated by a reference voltage source 11,
A drive signal is given to the output disconnection circuit 12 according to the comparison result. The output cutoff circuit 12 turns on / off a signal light output by turning on / off a pump light source (not shown) (not shown) of the optical amplification unit 2 in accordance with a drive signal given from the comparator 10. .

Next, the operation of the above configuration will be described.
Now, it is assumed that a wavelength shift occurs in an optical transmitter connected in front of the optical amplifier in FIG. 1 and the wavelength of the signal light gradually shifts from λ1.

When the wavelength of the signal light incident on the optical amplifier of FIG. 1 shifts from λ1, the amount of light transmitted through the optical filter 5 gradually decreases. Thereby, the Pin photodiode 6
, The intensity of the signal light incident on the logarithmic amplifier 9 decreases, and the amplified output of the logarithmic amplifier 9 decreases. On the other hand, the light incident on the Pin photodiode 4 is given to the logarithmic amplifier 7 with the same intensity because it does not pass through the filter. As a result, the intensity difference of the input signal to the differential amplifier 8 increases, and the differential amplifier 8
Output increases.

When the output of the differential amplifier 8 exceeds the threshold voltage, the comparator 10 outputs a drive signal to operate the output disconnection circuit 12 to turn off the pump light source of the optical amplifier 2. As a result, the amplification fiber becomes a light absorbing medium, and the optical signal output from the optical amplification unit 2 is immediately cut off.

According to the above configuration, when the wavelength of the incident light to be amplified changes, the operation of the optical amplifier 2 is automatically stopped. When this is applied to the configuration of FIG. 12, even if the output light wavelength of the optical transmitter connected to the preceding stage shifts due to aging or deterioration, the signal light is cut at the stage of the optical amplifier. As a result, the wavelength-shifted signal light does not mix with the wavelength-division multiplexed light, so that it is possible to prevent interference between signals, and to suppress deterioration of transmission quality due to deterioration of crosstalk between channels.

Of course, the same effect can be obtained when replacing the optical transmitter. That is, the wavelength of the output light shifts until the temperature of the laser light source of the replaced optical transmitter becomes stable, but until that time, the signal light is cut by the optical amplifier connected to the subsequent stage. Thereby, crosstalk between channels can be prevented.

As described above, in the present embodiment, the optical signal transmitted from the optical transmitter is partially branched by the directional coupler 1 before being introduced into the optical amplifier 2, and the optical signal is further divided by the directional coupler 3. To divide into two systems. The optical signal of one system is directly guided to the Pin photodiode 4 for photoelectric conversion, and the optical signal of the other system is guided to the Pin photodiode 6 via the optical filter 5 for photoelectric conversion. Pin
The photoelectric conversion outputs of the photodiodes 4 and 6 are used as the differential amplifier
And the comparator 10 compares the amplified output with the threshold voltage provided by the reference voltage source 11. As a result, when the output of the differential amplifier 8 exceeds the threshold, the output disconnection circuit 12 The optical amplifier 2 is turned off.

With this configuration, even if the output light wavelength of the optical transmitter fluctuates, crosstalk between channels can be prevented from deteriorating, and deterioration of transmission quality can be prevented. In addition, since signal light having a wavelength that should not be transmitted is automatically cut, the need for manual wavelength monitoring is eliminated, and human load can be reduced.

The operation of the optical amplifier having the above configuration is not affected by the fluctuation of the power of the optical signal input from the optical transmitter. This is because the optical signal is branched into two by the directional coupler 3, and the optical signal passing through the optical filter 5 and the optical signal not passing through are compared. In addition, the sensitivity to the wavelength shift amount can be freely set by appropriately setting the transmission characteristics and the threshold voltage of the optical filter 5.

(Second Embodiment) Next, a second embodiment of the present invention will be described. FIG. 2 is a block diagram illustrating a configuration of the optical amplifier according to the present embodiment. In FIG. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals, and here, only the different parts will be described.
Optical switch drive circuit 1 in place of comparator 10 having the configuration of FIG.
The optical switch drive circuit 14 turns on / off the optical switch 13 to cut off the output light of the optical amplifier 2 at the time of wavelength shift.

According to the configuration of FIG. 2, when the output of the comparator 10 is turned on by the wavelength shift, the optical switch driving circuit 14
Is driven, and the optical path in the optical switch 13 is switched to the signal off side. As a result, the output of the optical amplifier 2 is cut off, and the shifted wavelength is not output to the transmission line.

According to the above configuration, deterioration of crosstalk between channels due to wavelength shift of the optical transmitter can be avoided, deterioration of transmission quality can be suppressed, and human load can be reduced.

[Modification of Second Embodiment] FIG. 3 shows a modification of the optical amplifier of the present embodiment. As shown in the figure, the optical amplifier 2 may be positioned on the input side of the directional coupler 1 to turn on / off the output light of the directional coupler 1.

(Third Embodiment) Next, a third embodiment of the present invention will be described. FIG. 4 is a block diagram illustrating a configuration of the optical amplifier according to the present embodiment. The optical amplifier shown in FIG. 6 does not include the directional coupler 3 shown in FIG. 2, and directly guides the branched light from the directional coupler 1 to the optical filter 5.

According to this configuration, the function and configuration for detecting the wavelength shift can be simplified as compared with the configurations shown in FIGS. 1 to 3, and the size and weight of the wavelength division multiplexing optical transmitter can be reduced. It is possible to encourage.

[Modification of Third Embodiment] FIG. 5 shows a modification of the optical amplifier of this embodiment. As shown in the figure, an optical filter 5 may be provided between the directional coupler 1 and the optical amplifier 2. With this configuration, in addition to the effects described above, it is possible to cut ASE noise generated in the optical amplifier 2. That is, a unique effect of reducing noise can be obtained.

(Fourth Embodiment) Next, a fourth embodiment of the present invention will be described. FIG. 6 is a block diagram illustrating a configuration of the optical amplifier according to the present embodiment. The optical amplifier of FIG. 6 realizes the optical amplifier according to the concept of the present invention in the simplest form, and compares the signal light intensity via the optical filter 5 with the threshold voltage of the reference voltage source 11. It was done. With this configuration, the same effect as described above can be obtained.

As described above, by using the optical amplifiers shown in the first to fourth embodiments in place of the optical amplifiers 201 to 20n in FIG. 12, an optical signal of an unintended wavelength is mixed into the wavelength multiplexed light. Will be eliminated. As a result, crosstalk between channels can be kept good, and deterioration of transmission quality can be prevented.

(Fifth Embodiment) Next, a fifth embodiment of the present invention will be described. In the first to fourth embodiments, the optical amplifier connected to the subsequent stage of the optical transmitter has been described.
In the fifth embodiment, the configuration of the optical amplifying unit 2 in FIGS. 1 to 6 and its operation and effects will be described. The optical amplifier 2 described in this embodiment can be used not only for being installed in the optical amplifier shown in FIGS. 1 to 6, but also applicable as a general optical amplifier device.

FIG. 7 shows the configuration of the optical amplifier 2 according to the present embodiment. The optical amplifier 2 includes a pump laser diode 25, a laser drive circuit 26 for driving the laser diode 26, a multiplexer 22, and an optical fiber 23 for amplification. An optical fiber amplifier that amplifies light. Optical isolators 21 and 24 are provided on the input side of the multiplexer 22 and the output side of the amplification optical fiber 22, respectively, to prevent noise due to return light.

Further, as a feature of the present invention, the optical amplifying unit 2 includes a duplexer 28, an optical / electrical converter 29, an error amplifying circuit 210, and a PWM (Pulse Width Modulation) modulation circuit 27. I have. After the amplified light is partially branched and subjected to optical / electrical conversion, the signal level is compared with a predetermined reference voltage value, and the laser drive circuit 26 is driven according to the difference to drive the pump light from the pump laser diode 25. Is taken out.

Here, the laser driving circuit 26 is, for example, as shown in FIG.
Is driven based on a pulse width modulated drive signal supplied from the PWM modulation circuit 27. That is, the drive signal from the PWM modulation circuit 27 is given to the switching transistor 262 via the operational amplifier circuit 261, and the switching transistor 262 is turned on / off.
The pump laser diode 25 is turned on / off in response to turning off. As a result, a pulse-like drive current (collector current Ic of the switching transistor 262) having a predetermined duty ratio is applied to the pump laser diode 25, and the pump light guided to the amplification optical fiber 23 becomes pulse-like.

In this embodiment, the PWM modulation circuit 2
The modulation frequency of 7 is several hundred times faster than the lifetime (hereinafter referred to as τ) of rare earth element ions doped into the amplification optical fiber 23. For example, when a general erbium (Er) -doped optical fiber is used as the amplification optical fiber 23, the Er ion τ is 1 msec.
Therefore, the modulation frequency of the PWM modulation circuit 27 is set to 100 kHz or more.

In general, the gain of an optical amplifier is determined by the average pump light power within τ incident on an amplification optical fiber. That is, the on / off of the excitation light emitted from the excitation laser diode 25
If the off-frequency is higher than τ, a gain completely independent of the modulation pulse pattern and the instantaneous fluctuation of the pump light output can be obtained. Utilizing this, in the present embodiment, the gain of the optical amplifier 2 is controlled by controlling the average value of the output power within τ of the pump laser diode 25.

In order to control the average value of the output power within τ of the pump laser diode 25, the pump laser diode 2
5 by controlling the duty ratio of the on / off pulse. This principle will be described with reference to FIG. The duty ratio of the pulse signal is defined as a ratio (unit: percent) between the peak value of the pulse and the average value.

10A to 10C, the peak value of the pulse corresponds to the output power E of the pump laser diode 25 when the switching transistor 262 is turned on.
The average value corresponds to the average power Eav of the pumping light incident on the amplification optical fiber 23 within τ. Here, the output (E) of the pump laser diode light when the transistor 262 is turned on is set to a value larger than the output during continuous oscillation. Preferably, it is desirable to supply the pump laser diode 25 with a current that is obtained by subtracting the derating amount from the maximum rated current.

FIGS. 10A, 10B, and 10C show pulse duty ratios of 50%, 100%, and 10%, respectively.
And the area of the shaded portion corresponds to Eav. Since Eav changes in accordance with the duty ratio, and Eav is directly proportional to the amplification gain, it can be seen that the amplification gain can be controlled by controlling the duty ratio.

As described above, in the present embodiment, the drive current of the pump laser diode 25 is subjected to PWM modulation, and the pulse-like pump light is guided to the amplification optical fiber 23 to perform optical amplification. At this time, the output light level is monitored by the optical / electrical converter 29, the average laser power to the amplification optical fiber is controlled by controlling the pulse duty ratio by performing loop control, and the amplification gain is kept stable. Like that.

That is, by variably controlling the duty ratio of the pulse of the drive signal to the laser drive circuit 26, the average value of the output power within τ of the pump laser diode 25 can be controlled. The amplification gain of the section 2 can be controlled. By taking advantage of this,
By feeding back the output level from the electric converter 29 to the PWM modulation circuit 27, the duty ratio is reduced / increased in accordance with the increase / decrease of the amplification gain, and the amplification gain can be kept constant. Become.

With the above configuration, the pump laser diode 25 is affected by mode hopping and return light.
Even if the output fluctuates instantaneously, the instantaneous fluctuation will not be seen from the amplification optical fiber 23 side. That is, the instantaneous level fluctuation of the pump light is absorbed by the change of the output pulse, and the total average power incident on the amplification optical fiber 23 becomes constant. For this reason, it is possible to suppress the gain fluctuation of the amplification optical fiber 23 and to operate the optical amplification unit 2 stably.

On the other hand, the conventional optical amplifier has a configuration as shown in FIG. That is, the loop output from the error amplification circuit 210 is directly supplied to the laser drive circuit 26,
The gain control is performed by directly varying the output light level of the pump laser diode 25 that continuously oscillates. However, in the above configuration, since the stability of the amplification gain is directly affected by the stability of the output of the pump laser diode 25, the following problem occurs.

(1) When light emission near the threshold value of the pump laser diode 25 is performed, operation in a region where natural light emission is switched to laser oscillation is inevitable. Will fluctuate.

(2) The pump laser diode 25 includes:
An optical isolator for suppressing the reflected return light that causes mode hopping is required, and an optical isolator should be provided between the pump laser diode 25 and the multiplexer 22. However, the characteristics of the optical isolator have wavelength dependence, and it is not possible to provide the optical isolator for every wave tone. In particular, Er
In a doped optical fiber amplifier, low-noise characteristics can be obtained by injecting pumping light having a wavelength of 0.98 μm into the amplifying optical fiber 23, but there is no optical isolator having good separation characteristics for a wavelength of 0.98 μm. Therefore, even if it is desired to use the pumping laser diode 25 that emits pumping light having a wavelength of 0.98 μm for noise reduction, the wavelength and the previous output fluctuate due to mode hopping caused by the reflected return light, and noise is also generated. . In addition, even a slight change in the output of the pump laser diode 25 causes the gain of the optical amplifier to fluctuate, so that the output level and the noise figure become unstable, leading to a decrease in transmission quality in an optical transmission system incorporating the optical amplifier. Become.

Since the wavelength fluctuation and the output fluctuation of the pump laser diode 25 occur at a very high speed, it is necessary to increase the speed of the loop control shown in FIG. However, since the response characteristics of the rare-earth doped fiber are slow in principle (for example, 0.2 to 10 msec for the Er-doped fiber), no matter how fast the loop control is made, the rare-earth doped fiber itself cannot follow and the wavelength fluctuation / output fluctuation. There was a problem that cannot be suppressed sufficiently.

On the other hand, in the present embodiment, the drive current of the pump laser diode 25 is subjected to PWM modulation, and the pulse-like pump light is guided to the amplifying optical fiber 23 to perform optical amplification. Further, the output level from the optical / electrical converter 29 is fed back to the PWM modulation circuit 27, and the duty ratio of the pulse is reduced / increased in accordance with the increase / decrease of the amplification gain so as to keep the amplification gain constant.

By doing so, even if reflected return light or mode hopping occurs, the instantaneous level fluctuation of the pump light due to this can be absorbed by the change in the output pulse. It is possible to operate stably.

Accordingly, it is possible to provide an optical amplifier which eliminates the adverse effects of reflected return light and mode hopping without providing an optical isolator, and consequently provides a 0.98 μm
It is possible to promote the realization of an optical fiber amplifier using pump light of a wavelength, and it is possible to improve noise resistance.

Further, in this embodiment, it is possible to reduce power consumption and heat loss. This will be described with reference to FIG. 9 showing the VCE-IC characteristics of the switching transistor 262.

Generally, when a current flows through a transistor, internal power loss is converted into thermal energy, causing a temperature rise. The internal power loss of a transistor operating at a certain operating point is the sum of the collector loss (= Ic.VCB) and the emitter loss (= IE.VBE). In a normal case, since the emitter junction is forward-biased, VCB> VBE, and since ICＩIE, the power loss of the transistor is determined by the collector loss PC = IC · VCBＶIC · VCE.

Referring to FIG. 9, in the conventional gain control method using continuous oscillation, a current IA flows through the pump laser diode 25 in order to obtain a desired gain. Since the collector-emitter voltage at that time is VA, the collector loss of the transistor is PA = IA-VA. The operating point at this time is point A.

On the other hand, in the gain control method by PWM modulation according to the present embodiment, as described above, the current IB larger than the drive current IA during continuous oscillation is supplied to the pump laser diode 2.
Pour into 5. This increases the current that flows,
The voltage between the collector and the emitter can be much smaller. The operating point at this time is point B, and the collector loss is PB = IB.VB <PA.

That is, by adopting this embodiment,
The collector loss can be reduced, and as a result, the power loss can be reduced. Further, since the switching transistor 262 is turned on / off for turning on / off the pump laser diode 25, the collector loss becomes zero when the transistor is turned off. Therefore, macroscopically, it is possible to make the collector loss of the pump laser diode 25 much smaller than the gain control method in the conventional continuous oscillation.

This means that a structure for dissipating heat energy generated by power loss can be simplified. That is, it is possible to simplify the heat treatment mechanism such as the heat radiating plate and the fan, and to reduce the configuration scale of the module, and to promote the miniaturization and weight reduction of the optical amplifier and the wavelength division multiplexing optical transmitter.

As described above, since the pumping light is subjected to PWM modulation and the average power of the pumping light is controlled by the duty ratio, the pumping light can be stably incident on the amplifying optical fiber 23.

As a result, it is possible to avoid the fluctuation of the gain of the optical amplifier caused by the fluctuation of the oscillation wavelength of the pump laser diode 25 and the fluctuation of the output beforehand, and to obtain a stable gain characteristic. In summary, according to the present embodiment, the following effects can be obtained.

(A) Stable gain control can be performed irrespective of fluctuations in the output level of the pump laser diode 25 due to mode hopping. (B) Even in an optical fiber amplifier having no optical isolator for preventing return light, the gain of the pump laser diode 25 can be stabilized. (C) It is possible to reduce the collector loss of the switching transistor 262 in the laser drive circuit 26 that drives the excitation laser diode 26. (D) By reducing the collector loss in (C), the scale of the heat radiation process can be reduced, which is advantageous for miniaturization and weight reduction. Note that the present invention is not limited to the above embodiments.

For example, in place of the logarithmic amplifiers 7 and 9 shown in FIGS. 1 to 6 showing the first to fourth embodiments, amplifiers having general amplification characteristics may be used.

Further, in the fifth embodiment, the configuration based on the forward pumping system in which the pumping light enters the input stage of the amplification optical fiber has been described. The present invention is not limited to this, and the present invention similarly applies to a backward pumping type optical amplifier in which pumping light is guided from the output side of the amplification optical fiber to the amplification optical fiber, and to a bidirectional pumping type optical amplifier leading from the input and output sides. Applicable.

The concept of the present invention shown in the fifth embodiment can be similarly applied to an optical amplifier having no gain control loop. FIG. 11 is a block diagram in the case where the present invention is applied to an optical amplifier having no gain control loop. In other words, the PWM modulation circuit 27 applies pulse width modulation to pump light. Also in this manner, it is possible to reduce the collector loss of the switching transistor 262, reduce the heat loss, and contribute to the simplification of the configuration. In addition, various modifications can be made without departing from the spirit of the present invention.

[0073]

As described above in detail, according to the present invention, the optical amplifier is provided with the wavelength shift detecting means and the output disconnecting means, and the optical transmitter transmits an optical signal having a wavelength different from the pre-assigned transmission wavelength. In this case, the optical signal is turned off, so that crosstalk between channels can be prevented from deteriorating even if the output light wavelength of the optical transmitter fluctuates. It is possible to provide a wavelength division multiplexing optical transmission device and an optical amplifier in which quality degradation is suppressed.

Further, according to the present invention, the pump light is subjected to PWM modulation, and the average power of the pump light is controlled by the pulse duty ratio. Therefore, regardless of the output level fluctuation of the pump laser diode 25 due to mode hopping. Thus, stable gain control can be performed. In addition, it is possible to reduce the collector loss of the transistor that drives the excitation light source, reduce the heat loss, reduce the scale of the heat radiation process, and promote the reduction in size and weight.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a configuration of an optical amplifier according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a modification of the optical amplifier according to the second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of an optical amplifier according to a third embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of a modified example of the optical amplifier according to the third embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of an optical amplifier according to a fourth embodiment of the present invention.

FIG. 7 shows an optical amplifier 2 according to a fifth embodiment of the present invention.
FIG. 2 is a block diagram showing the configuration of FIG.

FIG. 8 is a circuit diagram showing a configuration of a laser drive circuit 26 of FIG. 7;

FIG. 9 shows VCE-I of the switching transistor 262.
The figure which shows C characteristic.

FIG. 10 is a principle diagram for explaining that the average value of the output power within τ of the pump laser diode 25 can be controlled by controlling the duty ratio of the on / off pulse of the pump laser diode 25.

FIG. 11 is a block diagram when the present invention is applied to an optical amplifier having no gain control loop.

FIG. 12 is a block diagram illustrating a configuration of a wavelength division multiplexing optical transmission device.

FIG. 13 is a diagram for explaining a problem caused by a wavelength shift of the optical transmitter.

FIG. 14 is a block diagram showing a configuration of a conventional optical amplifier.

[Explanation of symbols]

 1, 3 directional coupler 2 optical amplifier 4, 6 Pin photodiode 5 optical filter 7, 9 logarithmic amplifier 8 differential amplifier 10 comparator 11 reference voltage source 12 output disconnection circuit 13 ... optical switch 14 ... optical switch drive circuit 21, 24 ... optical isolator 22 ... combiner 23 ... amplifying optical fiber 25 ... excitation laser diode 26 ... laser drive circuit 27 ... PWM modulation circuit 28 ... demultiplexer 29 ... light / Electric converter 210: Error amplification circuit 261: Operational amplifier circuit 262: Switching transistor

Claims (17)

[Claims] 1. A plurality of optical transmitters to which transmission wavelengths different from each other are assigned, and a plurality of optical transmitters provided corresponding to each of the plurality of optical transmitters, amplifying optical signals transmitted from each optical transmitter. A wavelength-division multiplexing optical transmission device comprising: a plurality of optical amplifiers for outputting; and a wavelength multiplexer for wavelength-multiplexing the output light from the plurality of optical amplifiers and transmitting the multiplexed light to an optical transmission line. An optical amplifier provided in an optical waveguide extending from an input port to an output port, and amplifying an optical signal transmitted from the optical transmitter to be connected to the optical waveguide through the input port; and A branching unit for partially branching out the passing optical signal, and comparing the wavelength of the branched light extracted by the branching unit with the transmission wavelength assigned to the optical transmitter of the connection partner, and a shift between these wavelengths. Ah Wavelength multiplexing comprising: a wavelength shift detecting unit that outputs an output disconnection signal in a case; and an output disconnection unit that turns off an optical signal output from the optical amplification unit when the output disconnection signal is given. Optical transmitter. 2. The light-to-electricity converter for optically / electrically converting the split light taken out by the splitting means, the wavelength shift detecting means being interposed between the optical / electrical converter and the splitting means. And an optical filter that sets the pass band to a transmission wavelength allocated to the optical transmitter of the connection partner, and compares the output level of the optical / electrical converter with a predetermined threshold voltage. 2. The wavelength according to claim 1, further comprising comparison means for providing said output disconnection signal to said output disconnection means when an output level of said optical / electrical converter is lower than said threshold voltage. Multiplexed optical transmitter. 3. The optical system according to claim 1, wherein the wavelength shift detecting means is provided between the input port and the branching means, and has a pass band set to a transmission wavelength allocated to the optical transmitter of the connection partner. A filter, an optical / electrical converter for performing optical / electrical conversion of the branched light extracted by the branching unit, and comparing the output level of the optical / electrical converter with a predetermined threshold voltage, 2. A wavelength division multiplexing optical transmission according to claim 1, further comprising: comparing means for giving said output cutoff signal to said output cutoff means when an output level of the power converter becomes equal to or lower than said threshold voltage. apparatus. 4. The wavelength shift detecting means includes: a distributing means for distributing the split light taken out by the splitting means into two systems; and first and second photoelectric conversion means for optically converting the optical signals distributed by the distributing means. And a transmission wavelength allocated between the first or second optical / electrical converter and the distribution means and assigned to the optical transmitter of the connection partner. An optical filter having its pass band set to the output signal of the first and second optical / electrical converters, and a differential amplifier for providing an output corresponding to a difference between these output signal levels; The output level of the differential amplifier is compared with a predetermined threshold voltage, and when the output level of the differential amplifier becomes equal to or less than the threshold voltage, the output disconnection signal is given to the output disconnection unit. And comparing means. Wavelength multiplexing optical transmission apparatus according to claim 1. 5. The optical amplifying means includes an excitation light source, and amplifies the optical signal by guiding the excitation light generated by the excitation light source together with the optical signal from the optical transmitter of the connection partner to a rare-earth-doped optical fiber. Wherein the output disconnection means, when the output disconnection signal is given, turns off the optical signal output from the optical amplification means by stopping the driving of the excitation light source. The wavelength division multiplexing optical transmission device according to claim 1. 6. The output disconnecting means includes an optical switch provided in the optical waveguide for opening and closing a path of an optical signal passing through the optical waveguide, and the output switch is provided when the output disconnection signal is given. 2. The wavelength division multiplexing optical transmission device according to claim 1, wherein an optical signal output from the optical amplification unit is turned off by opening an optical switch. 7. An optical amplifier connected to an optical transmitter to which a predetermined transmission wavelength is assigned and amplifying output light of an optical transmitter to be connected, provided in an optical waveguide from an input port to an output port. Optical amplification means for amplifying an optical signal sent from the optical transmitter of the connection partner to the optical waveguide through the input port, and a branching means for partially extracting and extracting an optical signal passing through the optical waveguide, A wavelength shift detecting unit that compares the wavelength of the split light extracted by the splitting unit with the transmission wavelength assigned to the optical transmitter of the connection partner, and outputs an output disconnection signal when there is a shift in these wavelengths. And an output disconnecting means for turning off the optical signal output from the optical amplifying means when the output disconnection signal is given. 8. The wavelength shift detecting means, an optical / electrical converter for performing optical / electrical conversion of the branched light extracted by the branching means, and an optical / electrical converter interposed between the optical / electrical converter and the branching means. And an optical filter that sets the pass band to a transmission wavelength allocated to the optical transmitter of the connection partner, and compares the output level of the optical / electrical converter with a predetermined threshold voltage. 8. The light according to claim 7, further comprising: comparing means for providing the output disconnection signal to the output disconnection means when an output level of the optical / electrical converter becomes equal to or less than the threshold voltage. amplifier. 9. The optical system according to claim 1, wherein the wavelength shift detecting unit is provided between the input port and the branching unit, and has a pass band set to a transmission wavelength assigned to the optical transmitter of the connection partner. A filter, an optical / electrical converter for performing optical / electrical conversion of the branched light extracted by the branching unit, and comparing the output level of the optical / electrical converter with a predetermined threshold voltage, 8. The optical amplifier according to claim 7, further comprising: comparing means for supplying the output disconnection signal to the output disconnection means when the output level of the power converter becomes equal to or lower than the threshold voltage. 10. The wavelength shift detecting means includes: a splitting means for splitting the split light extracted by the splitting means into two; and a first and a second means for photoelectrically converting an optical signal split by the splitting means. And a transmission wavelength allocated between the first or second optical / electrical converter and the distribution means and assigned to the optical transmitter of the connection partner. An optical filter having its pass band set to the output signal of the first and second optical / electrical converters, and a differential amplifier for providing an output corresponding to a difference between these output signal levels; The output level of the differential amplifier is compared with a predetermined threshold voltage, and when the output level of the differential amplifier becomes equal to or less than the threshold voltage, the output disconnection signal is given to the output disconnection unit. And comparing means. The optical amplifier of claim 7. 11. The optical amplification means includes an excitation light source,
An optical fiber amplifier for amplifying the optical signal by guiding the excitation light generated by the excitation light source to the rare-earth-doped optical fiber together with the optical signal from the optical transmitter of the connection partner; and The optical amplifier according to claim 7, wherein when a signal is given, the optical signal output from the optical amplifying unit is turned off by stopping the driving of the excitation light source. 12. The output disconnecting means includes an optical switch provided in the optical waveguide for opening and closing a path of an optical signal passing through the optical waveguide, and the output switch is provided when the output disconnection signal is given. 8. The optical amplifier according to claim 7, wherein an optical signal output from said optical amplifier is turned off by opening an optical switch. 13. An amplifying optical fiber comprising a pumping light source and a rare earth-doped amplifying optical fiber, wherein the pumping light generated by the pumping light source is guided to the amplifying optical fiber to amplify an optical signal passing through the amplifying optical fiber. An optical amplifier, comprising: pulse modulation means for performing pulse modulation on pump light guided to the amplification fiber, and varying a duty ratio of a pulse of the pump light to control average pump light power in the amplification optical fiber. An optical amplifier characterized in that a desired amplification gain is obtained by doing so. 14. A feedback loop for variably controlling a duty ratio of a pulse of the pump light according to a level of the amplified light transmitted from the amplification optical fiber so as to keep the level constant. Claim 1
4. The optical amplifier according to 3. 15. The pulse modulating means includes: a pulse generating means for generating a pulse signal; and a light source driving means for turning on / off the excitation light source in accordance with the pulse signal given from the pulse generating means. 14. The duty ratio of the pulse of the excitation light is variably controlled by changing the duty ratio of the pulse signal generated by the pulse generation means.
Or the optical amplifier according to 14. 16. The light source driving unit turns on / off a driving current to the excitation light source according to the given pulse signal.
The optical amplifier according to claim 15, further comprising a switching transistor that turns off, wherein the drive current supplied by the switching transistor is increased at least than a current value necessary for continuously oscillating the pump light source. . 17. The modulation frequency of said pulse modulation is at least 100 times as long as the lifetime of said rare earth ions.
The optical amplifier according to claim 13, wherein the number is twice or more.