JP3538987B2 - Optical level monitoring module and optical level monitoring circuit - Google Patents

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 an electric amplifier.
The present invention relates to a light level monitoring module and a light level monitoring circuit for monitoring an output light level and a reflected light level at an output of a light conversion circuit. The present invention relates to a light level monitoring module and a light level monitoring circuit performed by a diode.

[0002] In recent years, the importance of optical amplifiers in optical communication systems has increased, and their application area has been steadily expanding. The first reason is that there is no quality deterioration due to conversion because it is not converted to electricity, the second is that there is no restriction on the operating speed in the electric circuit because it is not converted to electricity, and the third is that it is necessary for the electric circuit The fourth is the ability to build a bit-rate-free system because no clock is required, and the fourth is that the output light level can be increased to extend the relay distance, and the hope is that a completely new soliton communication will be realized.

In optical communication, the output light level is the most important factor that determines the system configuration and communication quality, so it is necessary to monitor the output light level. In particular, when an optical amplifier is used, the output light level is monitored. Because of the high level, it is also necessary to monitor the reflected light level due to connector disconnection or fiber breakage. Further, even in a normal electro-optical conversion circuit, it is not possible to detect a failure such as disconnection of the connector on the transmission side only by monitoring the output light level. Therefore,
An optical level monitoring module and an optical level monitoring circuit that monitor an output light level and a reflected light level at an output of an optical amplifier or an electro-optical conversion circuit are applied.

[0004]

FIG. 8 shows a configuration of a conventional optical amplifier.
In FIG. 8, reference numeral 51 denotes a pre-stage module, 52 denotes an optical amplifier, 53a denotes a post-stage module, 54 and 54a denote excitation laser diodes, and 55 denotes an automatic level control circuit (ALC).
Circuit), 56 is a light level monitoring circuit, and 58 is an alarm detection circuit. In FIG. 8, an automatic temperature control circuit for controlling the temperature of a commonly used excitation laser diode and a current for converting the output current of the photodiode into a voltage are shown.
The illustration of the voltage conversion circuit is omitted.

In the preceding-stage module, reference numeral 511 denotes an optical coupler that branches a part of the input light and supplies it to a photodiode 513 described later. Reference numeral 512 denotes a pump light and a reflected amplified signal light as viewed from an optical amplifier. An isolator 513 which terminates non-reflection and emits light in a direction different from the optical axis of the signal light from the input side is a photodiode which monitors the input light level by detecting a part of the input light.

[0006] The optical amplifying section is usually constituted by an erbium-doped optical fiber 521. In the latter-stage module, reference numeral 531 denotes an optical coupler for coupling the pump light to the erbium-doped optical fiber, and reference numeral 532 denotes a non-reflection termination of the reflected light from the output side as viewed from the output side, and furthermore, the reflected light to the optical coupler 531.
An isolator that radiates in a direction different from the optical axis at
An optical coupler 531a branches a part of the output light and supplies it to a photodiode 534 described later, and an optical coupler 533 branches a part of the reflected light and supplies it to a photodiode 534a described later. A total reflection element that totally reflects the output light and supplies it to a photodiode 534 described later, 534 is a photodiode that detects the output light level, 534a is a photodiode that detects the reflected light level, and 533a is an excitation laser diode 5 that is described later.
A total reflection element 535 for totally reflecting the output of the laser 4a is an optical coupler 531 that converts the output light of the excitation laser diode 54 and the excitation laser diode 54a, which will be described later, whose 90-degree polarization planes are different from each other.
Is a polarization splitter supplied to. Here, the optical coupler 531
a, the total reflection element 533, the photodiode 534, and the photodiode 534a constitute a light level monitoring module.

The ALC circuit includes a control voltage generation circuit for generating a voltage corresponding to the output light level detected by the photodiode 534 and a reference voltage, and an LD drive circuit for driving a laser diode (LD). .

The optical level monitoring circuit will be described later. The alarm detection circuit includes a photodiode 513 for detecting an input light level, a photodiode 534 for detecting an output light level, and a photodiode 5 for detecting a reflected light level.
34a, an abnormality in the pump laser diodes 54 and 54a is detected and displayed.

FIG. 9 shows a conventional optical level monitoring module. In FIG. 9, 1 is an optical coupler, 4 is a total reflection element,
Reference numeral 3 denotes a photodiode for detecting an output light level, and reference numeral 3a denotes a photodiode for detecting a reflected light level. The coupler film 11 is formed on the optical coupler, and the total reflection film 41 is formed on the total reflection element.

[0010] Incidentally, in FIG. 9, the reference numerals are assigned to the optical level monitoring module, and therefore do not match the reference numerals in FIG. 8. If we describe those relationships just in case, we can see in Figure 9
1 is 531a in FIG. 8, 3 is 534 in FIG.
3a corresponds to 534a in FIG.

The signal light amplified by the optical amplifying unit enters the optical level monitoring module from the left in FIG. 9 and is branched by a coupler film provided on the optical coupler, and a part thereof is directed toward the total reflection element. Most of the light is emitted to the right side of FIG. 9 as output light of the optical amplifier. Further, the light branched toward the total reflection element is reflected by a total reflection film provided on the total reflection element and guided to the photodiode 3.

On the other hand, the reflected light resulting from the disconnection of the optical connector on the output side or the disconnection of the optical fiber is incident on the optical level monitoring module from the right side in FIG. 9, and a part thereof is formed by the coupler film provided on the optical coupler. Photodiode 3a
It is led to. The remaining part of the reflected light that passes through the optical coupler and is emitted toward the optical amplifier on the right side of FIG. 9 does not return to the optical amplifier due to the action of the isolator provided in the subsequent module. In FIG. 9, illustration is omitted.

FIG. 10 shows a conventional optical amplifier mainly showing an optical level monitoring circuit. In FIG. 10, reference numeral 51 denotes a first-stage module, 52 denotes an optical amplifier, and 53a denotes a second-stage module. In the latter module, only the photodiode 534 for detecting the output light level and the photodiode 534a for detecting the reflected light level are shown. 5
4 is a pump laser diode, 55 is an ALC circuit, 57
Is a current-voltage conversion circuit (I / V) for converting the current output from the photodiode 534 to a voltage, and 57a is an I / V for converting the current output from the photodiode 534a to a voltage.
V and 56a are optical level monitoring circuits. The ALC circuit 55 includes an LD drive circuit 551 and a control voltage generation circuit 552.
a. Lastly, the light level monitoring circuit 56a detects an abnormality in the reflected light level and stops the LD drive circuit, and includes two logarithmic amplifiers 566 and 566a;
A subtraction circuit 567 and a comparator 565 are provided.

In FIG. 10, the same reference numerals as those in FIG. 8 are used. In an optical level monitoring circuit, two I / V
Log-transform the output of The logarithmic conversion is performed so that the converted voltages can be compared by subtraction. (If an attempt is made to make a comparison with an exact number, a division circuit is required, and realization with a simple analog circuit is difficult. .). Then, whether to allow the operation of the LD drive circuit or to shut down the LD drive circuit is selected according to the output level of the subtraction circuit. Normally, when there is no reflected light, the voltage level output from the I / V 57 can be set about 20 dB higher than the voltage level output from the I / V 57a. On the other hand, when there is reflected light, the voltage level output from I / V 57 is 10 dB lower than the voltage level output from I / V 57a.
Can be set higher. Therefore, when the connector is normal, the DC level corresponding to the difference of 20 dB is output from the subtraction circuit, and when the connector is disconnected, 10 DC is output from the subtraction circuit.
Since the DC level corresponding to the difference B is output, the DC level corresponding to the difference of 15 dB is set as the reference voltage of the comparator, the reference voltage is supplied to the inverting input terminal, and the output of the subtraction circuit is output to the ratio inverting input terminal. When the connector is normal, a voltage close to the power supply voltage can be taken out from the comparator, and when the connector is disconnected, a voltage close to the ground potential can be taken out. If the output terminal of the comparator is connected to a terminal for controlling the pulse current and the bias current of the LD drive circuit via a diode whose cathode is on the comparator side and whose anode is on the LD drive circuit side, when the connector is normal, the output terminal is connected. Since the diode is off, the LD drive circuit is allowed to operate normally, and when the connector is disconnected, the diode is turned on and the LD drive circuit is shut down.

[0015]

As shown in FIGS. 9 and 8, a conventional light level monitoring module is provided with two photodiodes. Correspondingly, FIG.
As shown in (1), in order to operate the conventional optical level monitoring circuit, two I / Vs are required and two logarithmic amplifiers are required. Accordingly, the cost of the conventional optical level monitoring module is unavoidably high, and the configuration of the conventional optical level monitoring circuit is also complicated and the cost is high.

[0016] In order to solve such a problem, the present invention provides an optical level monitoring module which can realize the same function as one photodiode by using one photodiode, and an optical level monitoring circuit adapted to the optical monitoring module. With the goal.

[0017]

FIG. 1 shows an embodiment of the present invention. In FIG. 1, 1 is an optical coupler, 2 is an attenuating element, and 3 is a photodiode. The optical coupler is provided with a coupler film by a known technique on a surface opposite to a surface on which signal light is incident from the optical amplifying unit, out of two surfaces through which light is transmitted, and the attenuating element is provided with an incident light. The signal light from the optical amplifying unit having the attenuating film that is attenuated and reflected is incident on the optical level monitoring module from the left side of FIG. 1, and most of the signal light is transmitted through the coupler film of the optical coupler. 1 is output to the right output side. Part of the signal light from the optical amplifier, which is branched by the coupler film of the optical coupler, enters the attenuation film of the attenuation element, is attenuated, reflected, and reaches the optical coupler again. Most of this light passes through the coupler film of the optical coupler and enters the sensitive region of the photodiode. Therefore, the light that has exited the optical amplification section is attenuated approximately equal to the sum of the branch loss at the optical coupler and the attenuation (reflection loss) at the attenuating element and enters the photodiode.

On the other hand, for example, the level of the reflected light when the optical connector is disconnected is obtained by subtracting the return loss of Fresnel reflection from the level of the output light. This is partially branched by the coupler film of the optical coupler and is incident on the photodiode. Therefore, the reflected light that reaches the photodiode undergoes an attenuation equal to the sum of the return loss of Fresnel reflection and the branch loss in the optical coupler, and enters the photodiode.

Now, with the axis of the signal light from the optical amplifier as the horizontal reference, the surface of the optical coupler on which the signal light is incident and the surface on which the coupler film is formed are set at an angle of 45 degrees with respect to the horizontal axis. If the surface on which the attenuation film of the attenuation element is formed is set parallel to the horizontal axis, the position where a part of the signal light from the optical amplifier enters the photodiode and the reflected light It can be easily understood that the entry positions coincide.

FIG. 2 shows a normal state of the connector in the configuration of FIG.
It is a figure explaining the discrimination operation of abnormality. Now, the return loss of Fresnel reflection is about 14 dB when calculated using the refractive index of ordinary glass and the refractive index of air, so the branch loss at the optical coupler is about 10 dB, and the output light level of the optical amplifier is Is 0 dB, the level of the reflected light entering the photodiode is about -24 dB. On the other hand, the light that exits the optical amplifier, is branched by the optical coupler, is attenuated by the attenuating element, is reflected and then passes through the optical coupler and enters the photodiode,
The level is reduced by the branch loss of 10 dB and the amount of attenuation in the attenuation element. If this level is set to −34 dB, which is 10 dB lower than the level of the reflected light entering the photodiode, the amount of attenuation at the attenuation element may be set to 24 dB.

Incidentally, if the optical connector is properly connected, the loss in the connector is about 0.01 dB, and the return loss in this state is 30 dB. Therefore, the level of the reflected light entering the photodiode when the connector is properly connected is about -40 dB.

Therefore, when the connector is properly connected, the level of light entering the photodiode is approximately -34 dB in power sum, and when the connector is disconnected, the level of light entering the photodiode is approximately -24 dB in power sum. By monitoring the DC level output from the photodiode, it is possible to determine whether the connector is normal or not.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The case where the arrangement of the optical coupler and the attenuating element is the simplest has been described above for the sake of understanding, but the case where the angle of the coupler film of the optical coupler is shifted from 45 degrees is described. If the angle of the attenuating film of the attenuating element is shifted accordingly to be perpendicular to the axis of light traveling from the optical coupler to the attenuating element, the position at which the optical coupler is branched and emitted in the direction of the attenuating element And the position reflected by the attenuating element and again incident on the optical coupler can be matched. The coincidence of these two positions means that the position where the light reflected by the attenuation element is incident on the coupler film again coincides with the position where the signal light from the optical amplifier enters the coupler film, and is reflected by the attenuation film. This means that the position where the transmitted light is transmitted through the coupler film and enters the photodiode and the position where the reflected light when there is reflection from the output side is split by the coupler film and is incident on the photodiode match. I do. Therefore, it is not necessary to limit the arrangement of the optical coupler and the attenuation element as described above.

FIG. 3 shows a modification of the embodiment of the present invention. In FIG. 3, 3 is a photodiode, 6 is a first prism, and 6a is a second prism. The surface where the first prism and the second prism are in contact has an inclination of 45 degrees with respect to the axis of the light incident from the optical amplification unit, and a coupler film 61 is formed on the surface of the 45 degrees, The surface of the second prism on which the signal light is incident is perpendicular to the axis of the incident light, and
The upper surface of the second prism is parallel to the axis of the incident light, and the attenuation film 62 is formed on this surface.

A part of the light incident on the second prism from the optical amplifier is branched by the coupler film toward the attenuation film, and most of the light is transmitted through the coupler film to become output light. Most of the light split by the coupler film and reflected while being attenuated by the attenuating film enters the photodiode.
On the other hand, for example, a part of the reflected light generated when the connector comes off is partly branched toward the photodiode by the coupler film.

This situation is exactly the same as the case of the configuration of FIG. Therefore, assuming that the branch loss in the coupler film is 10 dB, the return loss of Fresnel reflection when the connector comes off is about 14 dB. In order to lower the level of the reflected light by 10 dB, the attenuation of the attenuation film may be set to about 24 dB.

The configuration shown in FIG. 3 has the same principle as the configuration shown in FIG. 1, but the optical coupler and the attenuating element must be separately configured in FIG. As a result, only one element may be used, which has the advantages of reduced size as an optical level monitoring module and easy adjustment of the angle between the coupler film and the attenuation film.

In FIG. 3, the arrangement of the first and second prisms has been described under specific conditions. However, the present invention is not necessarily limited to this, as described at the end of the description of FIG.

Although the configuration shown in FIG. 3 is a configuration in which the two prisms are bonded, it is needless to say that the same function can be realized even if the two prisms are separated.
Both the attenuation element and the optical coupler have arbitrary shapes. In any case, the optical coupler and the attenuation element may be arranged so that the light branched by the coupler film can be perpendicularly incident on the attenuation film.

FIG. 4 shows a second embodiment of the present invention. In FIG. 4, reference numeral 1 denotes an optical coupler having a coupler film 11, reference numeral 3 denotes a photodiode, and reference numeral 4 denotes a reflection element provided with a reflection film 41.

The feature of FIG. 4 is that a reflective element is used, and
The point is that the surface of the reflection element on which the reflection film is formed is inclined at a small angle θ radian with respect to the axis of the light incident from the optical amplifier.

Considering the axis of the light incident from the optical amplifier as a horizontal reference, the angle between the surface of the optical coupler on which the signal light is incident and the surface on which the coupler film is formed is 45 degrees with respect to the horizontal. In this case, the axis of the light that is partially branched by the optical coupler and travels toward the reflection element becomes vertical according to the above-described angle standard. By the way, since the surface of the reflective element on which the reflective film is formed has an inclination of θ with respect to the horizontal, the axis of the light traveling from the optical coupler to the reflective element is inclined by θ with respect to the perpendicular to the reflective film. ing. Therefore, the axis of the light reflected by the reflection film toward the optical coupler is inclined by 2θ from the axis of the incident light. now,
If the distance between the emission position from the optical coupler and the incidence position of the reflection element is L, the emission position of the light emitted from the optical coupler and the incidence position of the reflected light returning to the optical coupler to the optical coupler are θ.
If is small enough, it may be shifted horizontally by about 2Lθ. The position where this light is transmitted through the optical coupler and enters the photodiode may be shifted horizontally by 2Lθ as compared with the case where there is no inclination of θ if the thickness of the optical coupler is sufficiently small. Therefore, the level of light that is branched by the optical coupler and reflected by the reflective element and enters the photodiode can be controlled by setting θ.

On the other hand, for example, the position where the light reflected when the connector comes off and enters the photodiode is irrelevant to θ. Therefore, if θ is set appropriately, it is possible to discriminate between the normal and abnormal states of the connector.

FIG. 5 is a view showing a normal state of the connector in the configuration of FIG.
It is a figure explaining the discrimination operation of abnormality. For simplicity of understanding, the cross section of the light beam is square, and the sensitive area of the photodiode is also square. When θ is 0, the beam and the sensitive area completely overlap, and the beam shift due to θ shift Is assumed to be shifted in a direction parallel to the sides of the square. Assuming this, the amount of light received by the photodiode decreases in proportion to the absolute value of θ, and becomes 0 when 2Lθ becomes equal to the length of the side of the sensitive region of the photodiode.

In order to obtain the same level as described above, the beam may be shifted to a position where the 24 dB level decreases. Decreasing by 24 dB means that the overlap between the beam and the sensitive area only needs to be 4/1000, so 2Lθ may be set to be 996/1000 of the side length of the sensitive area. Since the length of the side of the sensitive area is about 400 microns, 2Lθ is actually 398 microns. Here, if L is 5 mm, θ is about 0.04.
Radians (about 2.2 degrees) may be used.

Here, the model with the simplest calculation has been described. However, even if the cross section of the beam and the sensitive region are square, the direction is not parallel to the direction of the sides, or the cross section of the beam and the sensitive region are circular. In this case, the light receiving level only decreases more rapidly than the absolute value of θ (dashed line in FIG. 5).
Thus, there is no change in that the amount of received light can be controlled by θ.

In FIG. 4, the coupler film is fixed at 45 degrees with respect to the axis of the incident light, and the attenuation film is inclined by θ with respect to the axis of the incident light. The attenuating film may be formed parallel to the axis of the incident light by tilting θ from 45 degrees.

Further, the arrangement of the optical coupler and the reflecting element is limited to a specific condition for easy understanding. However, even when the above condition is not satisfied, the light is reflected from the reflecting element and the photodiode is not reflected. And the position where the light reflected when the connector comes off is incident on the photodiode, and both positions can overlap the sensitive region of the photodiode. That is, if the angle of the reflection film with respect to the light that is branched by the optical coupler and travels toward the reflective element is shifted from the vertical by a small angle θ, the position at which the light traveling from the reflective element to the optical coupler enters the optical coupler and the position at which the light exits the optical coupler It is because it can be made to differ. Therefore, the arrangement of the optical coupler and the reflection element is not limited to the above.

FIG. 6 shows a modification of the second embodiment of the present invention. In FIG. 6, 3 is a photodiode, 6 is a first prism, and 6b is a third prism. And
The surface where the first and third prisms are in contact is adjusted to an angle of 45 degrees with respect to the light beam from the optical amplifier, and a coupler film 61 is formed on the surface at the angle of 45 degrees.
Also, the surface on which the signal light is incident on the third prism is perpendicular to the axis of the signal light, and the upper surface of the third prism is formed at an angle of θ with respect to the light beam from the optical amplifier, A reflection film 63 is formed on the upper surface.

The configuration shown in FIG. 6 has the same principle as the configuration shown in FIG. 3. However, in FIG. 3, the optical coupler and the reflection element must be separately provided, whereas the configuration shown in FIG. As a result, only one element may be used, which has the advantages of reduced size as an optical level monitoring module and easy adjustment of the angle between the coupler film and the reflection film.

In the description of FIG. 6, the coupler film is formed at 45 degrees with respect to the axis of the incident light, and the reflection film is inclined by θ with respect to the axis of the incident light. The reflective film may be inclined by θ and formed in parallel with the axis of the incident light.

Further, in the above, the arrangement of the first and second prisms and the like have been described under specific conditions. However, the present invention is not necessarily limited to this, as described at the end of the description of FIG.

Although the configuration shown in FIG. 6 is a configuration in which the two prisms are bonded, it goes without saying that the same function can be realized even if they are separated. 5 and 6 are so-called Michelson interferometers. Therefore, the state of interference between the two lights changes due to the difference between the optical path length of the output light branched by the coupler and the optical path length of the reflected light, and the amount of light incident on the photodiode changes. Since this may affect the determination of the amount of light, it is desirable to adopt a structure in which, for example, the position of the coupler element can be adjusted in the left and right directions in the drawing.

FIG. 7 shows an optical amplifier mainly showing the optical level monitoring circuit of the present invention. In FIG. 7, reference numeral 51 denotes a first-stage module, 52 denotes an optical amplifier, and 53 denotes a second-stage module. In the latter module, only the photodiode 534 for detecting the output light level and the reflected light level is shown. 54 is an excitation laser diode, 55 is AL
C circuit, 56 is an optical level monitoring circuit, 57 is a current-voltage conversion circuit (I / V) for converting the current output from the photodiode 534 into a voltage.

The ALC circuit 55 includes an LD drive circuit 551 and a control voltage generation circuit 552. The light level monitoring circuit includes a smoothing circuit including a resistor 561 and a capacitor 562, a logarithmic amplifier 563 for logarithmically converting the output voltage of the smoothing circuit, and a shutdown of the LD drive circuit corresponding to the level output by the logarithmic amplifier. Comparator 564
And

As described above, when the connector is normal, the logarithmic amplifier outputs a DC corresponding to -34 dB,
When the connector is disconnected, the logarithmic amplifier outputs a DC corresponding to -24 dB. Therefore, if a voltage corresponding to about −30 dB is supplied as a reference voltage to the non-inverting input terminal of the comparator, the output of the comparator becomes a voltage close to the power supply voltage when the connector is normal, and when the connector is disconnected, The output is a voltage close to ground. If the output terminal of the comparator is connected to a control terminal that determines a pulse current and a bias current of the LD drive circuit via a diode whose comparator side is a cathode and the LD drive circuit side is an anode, the diode is connected when the connector is normal. The operation of the LD drive circuit is allowed because the switch is off, and when the connector is disconnected, the diode is turned on and the LD drive circuit is shut down.

Although the logarithmic amplifier is used in the configuration shown in FIG. 7, it is assumed that the voltage level corresponding to the output light level and the voltage level corresponding to the reflected light level are both absolute levels. The ratio is a predetermined value, ie, dB
If the difference between the two is a predetermined value, the two can be discriminated from each other. If the difference between the two voltage levels can be maintained at a large value, a normal amplifier may be used.
If / V can output a sufficient voltage, it is not necessary to install an amplifier.

By the way, only the light path in the optical level monitoring module has been described so far, and the mounting technology of the optical level monitoring module and the forming technology of the coupler film, the attenuation film, and the reflection film have not been described. The reason for this is that these are all known technologies, but only the attenuating film not used in the conventional optical level monitoring module shown in FIG. 9 will be briefly described.

For example, a film of a substance B which is intermediate between the refractive indices of the substance A and air (normally, the refractive index is equal to the square root of the product of the refractive indices of the two substances) is attached to the substrate of the substance A, and light is emitted from the air side. When the light is incident, the light is reflected with a phase shift of 180 degrees at the interface between the air and the substance B and the interface between the substance B and the substance A. Therefore, if the thickness of the substance B is set to about 1/4 of the wavelength of the light, the light reflected at the interface between the air and the substance B, the substance B and the substance A
Since the difference in the optical path length of the light reflected at the interface becomes 180 degrees, the reflected lights in the air weaken each other and the light level becomes 0.
become.

Conversely, when a substance C having a higher refractive index than the substance A is attached to the substrate of the substance A and light is incident from the air side, the light is reflected at the interface between the air and the substance C with a phase shift of 180 degrees. At the interface between substance C and substance A, light is reflected in phase. Therefore, if the thickness of the substance C is about 1/4 of the wavelength of the light, the reflected light in the air is enhanced, and the light level is doubled.

Using the above two principles, an attenuation film can be formed. For example, when titanium dioxide and silicon dioxide films having different refractive indices are alternately deposited on a glass substrate with a thickness of about の of the wavelength and light is incident from the air side, the phase is shifted by 180 ° and reflected. And the interface that reflects without a phase shift can be formed alternately. As a result, the weakly reflected light and the strongly reflected light are combined in the air, and the level of the reflected light in the air can be controlled by appropriately selecting the number of layers. Incidentally, 2 in the description of FIG.
In order to obtain a return loss of about 4 dB, a four-layer film may be used when the above-mentioned substance is used.

[0052]

As described in detail above, according to the present invention, two photodiodes required in the conventional optical level monitoring module are reduced to one, and a circuit for converting the output current of the photodiode into a voltage correspondingly. And the number of logarithmic amplifiers can be reduced to one. This makes it possible to reduce the cost of the optical level monitoring module and the optical level monitoring circuit, and furthermore, the cost of the optical amplifier, and to further simplify the configuration of the optical level monitoring circuit and the optical amplifier.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention.

FIG. 2 is a diagram illustrating a normal / abnormal discriminating operation of the connector in the configuration of FIG. 1;

FIG. 3 is a modification of the embodiment of the present invention.

FIG. 4 shows a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a normal / abnormal discriminating operation of the connector in the configuration of FIG. 3;

FIG. 6 is a modification of the second embodiment of the present invention.

FIG. 7 is an optical amplifier mainly showing an optical level monitoring circuit of the present invention.

FIG. 8 shows a configuration of a conventional optical amplifier.

FIG. 9 shows a conventional optical level monitoring module.

FIG. 10 shows a conventional optical amplifier mainly showing an optical level monitoring circuit.

[Explanation of symbols] 1 Optical coupler 2 Attenuation element 3 Photodiode 11 Coupler film 21 Attenuation film

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshikazu Terayama 2-1-1 Kitaichijo Nishi, Chuo-ku, Sapporo-shi, Hokkaido Fujitsu Hokkaido Digital Technology Co., Ltd. (72) Inventor Kazuto Imai Chuo-ku, Sapporo, Hokkaido 2-1-1 Kita-Ichijo-Nishi Fujitsu Hokkaido Digital Technology Co., Ltd. (56) References JP-A-6-1311691 (JP, A) JP-A-7-325008 (JP, A) US Patent 4089552 (US, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H04B 10/08 H01L 31/0232

Claims (3)

(57) [Claims] 1. An optical coupler comprising a glass body having a coupler film formed on at least one surface thereof, an attenuating element comprising a glass body having an attenuated film formed on at least one surface, and one photodiode. The optical coupler and the optical coupler are coupled such that light incident on the optical coupler, branched by the coupler film, and emitted from the optical coupler toward the attenuating element is perpendicularly incident on the attenuating film formed on the attenuating element. An attenuating element, and the optical coupler and the photodiode are arranged such that light reflected by the attenuating element and incident again on the optical coupler is incident on a sensitive area of the photodiode. And light level monitoring module. 2. An optical coupler comprising a glass body having a coupler film formed on at least one surface thereof, a reflection element comprising a glass body having a reflection film formed on at least one surface, and one photodiode. Light incident on the optical coupler, branched by the coupler film, and emitted from the optical coupler toward the reflective element is incident on the reflective film formed on the reflective element at an angle shifted from the vertical by a small angle. The optical coupler and the reflective element are arranged such that light reflected by the reflective element and incident again on the optical coupler is incident on the sensitive region of the photodiode, and incident on the optical coupler. The optical coupler and the photocoupler are split by the coupler film so that light incident on the optical coupler from the direction of the axis of the other light output from the optical coupler enters the sensitive region of the photodiode. Optical level monitoring module, characterized in that arranging the diode. 3. An optical level monitoring circuit connected to the optical level monitoring module according to claim 1, wherein the current-voltage conversion circuit converts an output current of the photodiode into a voltage. Upon receiving the output voltage of the conversion circuit, if there is no reflection from the output side of the optical level monitoring module, generate a voltage that allows the operation of the laser diode drive circuit,
A light level monitoring circuit comprising: a comparator that generates a voltage that shuts down a laser diode drive circuit when there is reflection from an output side.