JP3349367B2 - Optical transmission module - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device for converting an electric signal into an optical signal in the field of optical communication for transmitting a signal using an optical fiber. The present invention relates to an optical transmission module that does not change.

[0002]

2. Description of the Related Art In recent years, digitization of signals, transmission of images,
New communication applications such as data communication between computers typified by the Internet have spread to ordinary offices and homes. In these applications, conventional telephones, FA
Compared with communication applications represented by X, the amount of data to be transmitted has been dramatically increased. For example, a band of about 16 kHz is good for a conventional analog telephone, but a band of about 1 MHz is required for an ISDN signal, which is a global standard for digital telephones. MPEG2, which is a global standard band compression method for digital images, requires a band of about 4 MHz.

In such a field, since it is necessary to transmit data to individual homes, an electrical transmission module is mainly used. In the electrical method, when the transmission band of the signal is widened, the transmission distance where the waveform is not deteriorated is shortened. For this reason, there is a problem in that the more a wideband signal is to be transmitted, the more relay devices need to be installed on the way.

On the other hand, in optical transmission, there is no problem of signal waveform deterioration up to a band of about GHz. Further, light entering the optical fiber can be transmitted very efficiently, which is extremely convenient for high-speed and long-distance signal transmission. However, there is a problem that an optical-electrical conversion device required for optical communication, that is, an optical transmission module is expensive and cannot be used for transmitting data to individual homes.

The above-mentioned conventional optical transmission module is expensive because light emitted from a light source typified by a semiconductor laser is made incident on an optical fiber and the amount of light entering the optical fiber, that is, the so-called light amount in the fiber is considered. It is a very difficult technology to keep it constant, and it is necessary to use special parts with small changes in characteristics due to temperature changes in the light source, condenser lens, case, etc. so that the amount of light in the optical fiber does not change due to temperature. No.

FIG. 5 shows an example of a conventional optical transmission module. In the conventional example, the adjustment and assembly of the optical component are performed at room temperature, and the position of the optical component is fixed. For this reason, in order to prevent a decrease in optical coupling efficiency due to deviation of the end face 86 of the optical fiber 84 from the best image point due to a temperature change, the condenser lens 8 is used.
Use a glass lens with a small change in the focal position as 2,
Case 85 for fixing the distance between the semiconductor laser 81 and the condenser lens 82 and the distance between the condenser lens 82 and the optical fiber end face 86
For example, stainless steel having a small linear expansion coefficient and being resistant to mechanical deformation is used. In this case, the stainless steel was not only expensive, but also difficult to weld and assembly of the module was not easy. Here, the optical coupling efficiency is the ratio of the light that reaches the fiber end face and is taken into the fiber. Reference numeral 83 denotes a fixture for fixing the optical fiber 84.

In FIG. 5, the output I of the monitor PD is set to a driving circuit 87 so that the output of the semiconductor laser is also constant.
And the driving current J of the semiconductor laser is controlled. One of the causes of the change in the amount of light in the optical fiber is that the output of the light source changes with temperature. As a light source, a small-sized semiconductor laser which can efficiently convert current into light is usually used, and its characteristics are known to change sensitively with temperature. In general, it is known that the temperature characteristics of the optical output of a semiconductor laser decrease as the operating temperature increases, and are proportional to the drive current at the same temperature. Therefore, in a semiconductor laser,
In order to obtain the same light output, it is necessary to increase the drive current as the temperature increases.

As a control method for keeping the light output constant when the temperature changes, the characteristics of each element are stored in a ROM (non-volatile memory) or the like, and the driving current of the semiconductor laser is changed according to the output of the temperature sensor. Is disclosed in Japanese Patent Publication No. Hei 8-31829, but this method requires a complicated control circuit and makes the optical transmission module expensive.

As another control method, since the semiconductor laser emits light not only on the optical fiber side but also on the opposite side, the amount of light unnecessary for optical transmission on the opposite side to the optical fiber is monitored, and the amount of light is monitored. A method of controlling a driving current so that the light amount is constant is well known. This method is usually an APC method (APC method is called Auto Power Control).
11 is an automatic output control method), and is the most frequently used method. In this method, a photodetector that monitors the amount of unnecessary light has a large light receiving area, does not need to be adjusted, and has a low response speed. For this reason, in the APC method, the average signal strength within the response time of the monitoring photodetector is controlled to be constant.

However, in the field of optical communication, it is not necessary to keep an optical signal at a constant intensity, and a state of "1" where light is emitted and a state of "0" where light is not emitted are repeated at high speed. . To keep the light output constant means to keep the light output in the state of "1" and "0" constant.
Within the response time of the monitoring photodetector, the number of "1" and
For a signal where the ratio of the number to "0" is almost constant,
The output of the monitoring photodetector is proportional to the light output in the "1" state. However, if the ratio of the number of "1" to the number of "0" changes during a time shorter than the response time of the monitor photodetector, the output of the monitor photodetector becomes the light in the "1" state. It is no longer proportional to the output, and it is difficult to stabilize the optical output in the state of “1” by the APC method.

In the prior art, extra data is added so that the ratio of the number of "1" s to the number of "0s" becomes almost constant. However, in recent years, without adding the extra data in order to increase the signal transmission efficiency, a signal in which the ratio of the number of “1” and the number of “0” changes during the time of averaging is used. Transmission systems are increasingly being used.

Another cause of the unstable light amount in the optical fiber is a distance between a light source and a means such as a lens for condensing light emitted from the light source, a light for transmitting the condensed light from the lens. The positional relationship of the optical component such as the distance from the fiber may change due to a change in temperature or the like. As a result, the shape of the focused spot on the end face of the optical fiber is initially deformed from the set shape, and the ratio of light incident on the optical fiber, that is, the so-called optical coupling efficiency changes.

In particular, when a single-mode fiber is used as the optical fiber, the light receiving diameter is as small as about 10 to 20 μm. Therefore, the diameter of the condensed spot on the end face of the optical fiber needs to be reduced to the same level. Focusing spot diameter W
Is given by: W = λ / NAf (1) where NAf is the numerical aperture NA of the optical fiber side of the condenser lens and λ is the wavelength of the semiconductor laser. Equation (1) is determined by the diffraction of light and is a value that cannot theoretically be made smaller. For example, λ = 1.
If 3 μm and NAf = 0.1, W = 13 μm.

On the other hand, the diameter of the core, which is the portion for confining the light of the single mode fiber, is up to 9 μm, but the light in the optical fiber partially seeps into the cladding region provided outside the core. Therefore, the light receiving diameter is 10 μm
Becomes

The optical coupling efficiency is determined by the degree of coincidence between the electric field of the light in the fiber and the electric field of the collected light at the end face of the fiber. That is, it is preferable that the size, intensity distribution, and the like of the converging spot be closer to the light intensity distribution in the optical fiber.

Another factor that determines the optical coupling efficiency is the magnitude of the angle of incidence on the optical fiber, and FIG. 6 is a diagram illustrating the relationship between the critical angle of incidence θmax on the optical fiber and the total reflection angle θc. . The light propagates while being reflected in the optical fiber. However, unless the incident angle is equal to or less than a certain angle θ _C , the light is not reflected and exits toward the clad. This angle θc is called the total reflection angle. Only light that enters the fiber within the range of the incident angle that is equal to or less than the total reflection angle propagates through the optical fiber. Assuming that the incident angle at which the total reflection angle θc is just in the fiber is the critical incident angle θmax, the optical coupling efficiency is determined by the ratio of the light having a critical incident angle or less to the total amount of light collected.

The critical angle of incidence θ _C and the NAf of the fiber have the following relationship: NAf = sin θmax (2)

In order to reduce the focal spot,
Equation (1) shows that the NA of the lens should be increased. However, since the NAf of a single mode optical fiber is about 0.1, even if the NA of the fiber side of the lens is made larger than 0.1, the mode does not become the propagation mode, and the fiber goes out of the fiber within several meters. I will go. The spot diameter determined by the expression (1) is an ideal value, and is realized only when the end face of the optical fiber is placed on the best image plane of the lens. If the distance between the lens and the fiber changes due to a change in ambient temperature or the like, the spot diameter becomes larger than the value of equation (1).

Normally, when the incident light is converged to near the limit given by the equation (1), the size of the condensed spot can greatly change even for a slight change in the positional relationship of the optical components. Are known. For this reason, the optical component is usually made of a material that does not easily change with temperature or the like. For example, the lens is preferably made of glass, and the material holding the optical system is preferably made of stainless steel. However, these materials are problematic in that they are expensive, difficult to process, and the like, and optical transmission modules cannot be manufactured at low cost.

[0020]

SUMMARY OF THE INVENTION A semiconductor laser light source;
In an optical transmission module including a means for condensing light emitted from the light source and an optical fiber for transmitting the condensed light, assembly / adjustment is easy, inexpensive, and in response to a change in operating temperature. And to obtain an optical transmission module having stable performance.

[0021]

An optical transmission module according to the present invention.
At least a semiconductor laser light source and the light source
Means for condensing light and light for transmitting the condensed light
An optical transmission module comprising: a fiber;
At the highest operating temperature range of the transmission module
In order to maximize the optical coupling efficiency to the optical fiber,
A source, and means for collecting light from the light source;
And an optical fiber for transmitting light, and
The amount of current injected into the semiconductor laser light source varies with temperature.
Means for coupling the optical fiber to the optical fiber.
It has an increasing characteristic with respect to a change in the degree of increase, and the light of the light source
The output has the characteristic of decreasing with increasing temperature
It is characterized by that.

[0022]

[0023]

[0024]

[0025]

[Operation] According to the present invention, in order to make the light quantity in the optical fiber constant irrespective of the temperature, the light coupling efficiency on the optical fiber is maximized at the maximum operating temperature of the optical transmission module. By setting, the decrease in optical coupling efficiency due to the decrease in temperature is compensated for by the increase in the optical output of the semiconductor laser used as the light source, and the above-mentioned amount of light in the fiber can be kept constant regardless of the temperature. A resin lens which is inexpensive but has a large change in focal position due to a change in temperature can be used as a condenser lens, and an optical transmission module can be manufactured at low cost. This makes it possible to easily install a signal transmission device capable of transmitting a wideband signal in each home or office.

A resin having a large linear expansion coefficient is used as a material of a case for fixing a positional relationship between individual optical components such as a distance between a light source and a condenser lens or a distance between a condenser lens and an end face of an optical fiber. Even in such a case, the light amount in the fiber can be made constant irrespective of the temperature by compensating by controlling the light output of the semiconductor laser used as the light source in response to the change in the optical coupling efficiency due to the temperature change. Can be manufactured at lower cost.

Further, by using a laser driving device having a circuit configuration that changes the driving current of the semiconductor laser depending on the temperature, the optical coupling in the case where the above-mentioned resin condensing lens, the resin case, or both are used. It is possible to prevent a decrease in the amount of light in the optical fiber due to a decrease in efficiency.

[0029]

1 to 4 are views showing an embodiment of the present invention. FIG. 1 is a diagram for explaining the operation of an optical transmission module, and FIG. FIG. 2B is a diagram for explaining the operation characteristics of the optical transmission module, FIG. 2 is a diagram for explaining the operation of the optical transmission module of another example, and FIG. FIG. 3B is a diagram for explaining the operating characteristics of the optical transmission module. FIG. 3 is a graph showing the relationship between the current amount of the semiconductor laser used as the light source of the optical transmission module, the so-called relationship between the driving current and the optical output. FIG. 4 is a diagram showing the relationship between the operating temperature and the amount of change in the best image point position when a resin condensing lens is used in the optical transmission module.

Hereinafter, an example according to an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 3 shows the current amount of a semiconductor laser used as the light source of the optical transmission module according to one embodiment of the present invention, that is, the relationship between the drive current and the light output, measured at different operating temperatures. Temperature is -20 ° C, 0 ° C, 20
It is shown that as the temperature increases, the efficiency of the laser light output with respect to the drive current decreases, and the slope of the current-laser light output correlation line also decreases.

In FIG. 3, when the driving current of the semiconductor laser is kept constant at 21 mA, the curve A shows an operating temperature of 60 ° C.
And the optical output is 2 mW, but at an operating temperature of −20 ° C., the optical output greatly changes to 8.8 mW. On the other hand, a curve B shows an example of a characteristic in a case where the driving current is controlled by the operating temperature so that the optical output becomes 2 mW. The driving current is 15 mA when the operating temperature is 40 ° C., and 21 mA when the operating temperature is 0 ° C. It is shown that. As can be seen from FIG. 3, in a semiconductor laser, in order to obtain the same light output, it is necessary to increase the drive current as the temperature increases.

Next, FIG. 1 showing an embodiment of the present invention will be described.
In the optical transmission module shown in FIG.
The light exiting from is collected by the aspherical glass lens 22. At this time, each optical component is adjusted and fixed while monitoring so that the light output from the single mode optical fiber 24 is maximized. 23 is an optical fiber fixing part. At this time,
The end face 26 of the optical fiber 24 is located at a position called the best image point, and the light spot shape on the end face 26 of the optical fiber has a diameter of 1.
It is a spot-like spot having a size close to the theoretical limit of about 4 μm. When the optical fiber end face 26 approaches or moves away from the best image point in the direction of the lens, the size of the light spot increases, the optical coupling efficiency decreases, and the optical fiber 24
The amount of light entering inside changes.

When the end face 26 of the optical fiber is shifted in the direction perpendicular to the direction of the lens, the mode in the optical fiber 24 and the intensity distribution of the converging spot deviate, so that the light supply efficiency is reduced. If 24 is tilted, the optical coupling efficiency is reduced by reducing the proportion of light entering the NAf of the fiber, but these factors can be prevented by careful assembly.

In FIG. 1B, a curve C is a graph showing the relationship between the operating temperature and the laser light output, a curve D is a graph showing the relationship between the operating temperature and the optical coupling efficiency, and a curve E is the operating temperature. 5 is a graph showing a relationship between temperature, laser light output, and optical coupling efficiency, that is, a graph showing the amount of light in a fiber, and corresponds to a product of a curve C and a curve D. In the example according to the embodiment of the present invention, the operating temperature range of the optical transmission module is -2.
It was designed from 0 ° C to 60 ° C. Each optical component is adjusted at the maximum operating temperature of 60 ° C., and the light output from the optical fiber 24 is maximized and fixed. Normal operating temperature, eg 25
At ℃, the focused spot size becomes larger than the value represented by the equation (1), and the light coupling efficiency changes as compared with the case of 60 ° C. due to the change of the focused spot size due to the temperature change.
As shown in curve D of FIG. That is, the temperature is 60 ° C.
Has an optical coupling efficiency of about 90%, an optical coupling efficiency of about 46% at a temperature of 40 ° C., and an optical coupling efficiency of about 35% at a temperature of 25 ° C.
%, And the optical coupling efficiency at a temperature of −20 ° C. is about 22%.

In the case of the present invention, the semiconductor laser 21 and the condenser lens (aspherical glass lens) 22, and the condenser lens 22 and the optical fiber end face 2 are made of inexpensive mild steel for the case 25.
Since the distance from the case 6 changed greatly as compared with the case where stainless steel was used for the case, a change in the optical coupling efficiency as shown by a curve D in FIG. 1B occurred. Further, the end face 26 of the optical fiber is fixed to the case 25 via a fixing part 23 such as a ceramic ferrule.

A curve C in FIG. 1 (b) is obtained by plotting a change in light output when the semiconductor laser emits light at a constant current with the light output at a temperature of 60 ° C. being 2 mW, with the temperature plotted on the horizontal axis. The light output at a temperature of 40 ° C. is about 4.7 mW, the light output at a temperature of 20 ° C. is about 6.8 mW,
The light output at ℃ is about 8.0 mW, and the light output at -20 ° C. is about 9.0 mW.

As described above, with respect to the operating temperature, the optical coupling efficiency increases as the temperature increases, while the laser light output decreases as the operating temperature increases. Therefore, these products can make the amount of light in the fiber substantially constant, as shown by the curve E in FIG. 1B.

FIG. 2 shows another example according to an embodiment of the present invention. In the optical transmission module of FIG.
1 is a semiconductor laser, 52 is a condenser lens (resin condenser lens), 54 is an optical fiber, 55 and 56 are cases, 57
Is the end face of the optical fiber. The operating temperature range of the optical transmission module was designed from 0 ° C. to 40 ° C. Each optical component is adjusted at the maximum operating temperature of 40 ° C., and the light output from the optical fiber is maximized and fixed. Normal operating temperature, for example, 25 ° C
Is larger than the value represented by the expression (1) as in the case of the first embodiment, and the light coupling efficiency is 40 ° C. In comparison, it decreases like the curve G in FIG.

In the embodiment of the present invention, a very inexpensive resin condenser lens is used as the condenser lens. In general, the resin expands as the temperature increases, and the refractive index decreases. As a result, when resin is applied to the lens, the focal length increases. FIG. 4 shows this state, and the focal length is extended by about 30 μm per 1 ° C.
The shape of the focused spot does not change much at the best image point position at each temperature. Therefore, if the end face 57 of the optical fiber can be brought to the best image point at each temperature, the optical coupling efficiency does not deteriorate.

FIG. 4 will be further described. This graph shows the relationship between the operating temperature and the amount of change in the best image point position when a resin condensing lens is used.
Then, the operating temperatures are 0 ° C., 20 ° C., 40 ° C., and 60 ° C.
At the time of ° C., the best image point position change amounts are -70 μm and 0
μm, 80 μm, and 162 μm.

In the example according to the embodiment of the present invention, the optical fiber end face 57 is moved in response to a large change in the focal position of the resin condensing lens 52.
Resin. Semiconductor laser 51 and condenser lens 5
Even if the distance between them is changed, the same effect as changing the distance between the condenser lens 52 and the optical fiber end face 57 is obtained. The coefficient of linear expansion of the resin is several to ten times larger than that of the metal, and is suitable for this application. Since the change in the focal length due to the temperature rise cannot be completely compensated for by the extension of the resin cases 55 and 56 alone, the above-described change in the optical coupling efficiency has occurred. By optimizing the optical coupling efficiency at the maximum operating temperature, it is possible to keep the amount of light in the fiber from decreasing so much as shown by the curve H in FIG.

FIG. 2B shows the relationship between the operating temperature and the optical output of the semiconductor laser, the relationship between the operating temperature and the optical coupling efficiency, and the operating temperature and the inside of the optical fiber in an example according to an embodiment of the present invention. This shows the relationship with the amount of light. In the case of FIG.
The optical output of the semiconductor laser is larger at 25 ° C. than at 40 ° C. When the light output when the semiconductor laser emits light at a constant current with the light output at 40 ° C. being 2 mW is plotted on the abscissa, the operating temperature increases as shown by a curve F in FIG. 2B. Since the amount of light entering the optical fiber is proportional to the product of the curve F and the curve G, the amount of light entering the optical fiber is 40 ° C. within the operating temperature range as shown by the curve H in FIG. Does not change so much. In the case of the embodiment according to the embodiment of the present invention, the drive current of the semiconductor laser 51 is set to slightly increase with the temperature in order to prevent a decrease in the amount of light in the optical fiber 54. At this time, the output of the semiconductor laser 51 is not monitored, but only the temperature is detected by a thermistor or the like (not shown), and the driving current of the semiconductor laser 51 is changed as shown by a curve B in FIG. Variations in characteristics due to the elements are realized by adjusting the slope of the curve B while observing the output of the optical fiber 54 at the time of inspection immediately after module assembly.

[0044]

According to the optical transmission module of the present invention,
The amount of current injected into the semiconductor laser light source changes with temperature
Characterized in that the optical fiber
Compensation for decrease in optical coupling efficiency to fiber by increasing optical output
To reduce the amount of light in the fiber over a wider operating temperature range.
It can be constant.

As described above, the decrease in the optical coupling efficiency due to the temperature change is considered.
The output of the semiconductor laser used as the light source is compensated for by the increase,
Expensive glass lens or expensive and difficult to process stainless steel
The light amount in the optical fiber can be used without using a case made of
Can be almost constant in the range, for example, inexpensive
Resin resin that has a large change in focal position due to temperature changes
Lens can be used as a condenser lens,
The transmission module can be manufactured at low cost. this thing
Signal transmission capable of transmitting broadband signals
The device can be easily installed in each home or office.
Become.

The magnitude of the change in length due to the change in temperature
The distance between the light source and the condenser lens or the condenser lens
Position of individual optical components, such as the distance between
The temperature can also be increased by using
To reduce the optical coupling efficiency due to the
Making optical transmission modules more inexpensive
Can be.

[0047]

[0048]

[0049]

[0050]

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams for explaining the operation of an optical transmission module according to an embodiment of the present invention, wherein FIG. 1A is a schematic cross-sectional view of the optical transmission module, and FIG. It is a diagram for explaining, a curve C is a graph showing a relationship between operating temperature and laser light output, a curve D is a graph showing a relationship between operating temperature and optical coupling efficiency, and a curve E is a graph showing operating temperature and laser light. 5 is a graph showing the relationship between the optical output and the optical coupling efficiency, that is, a graph showing the amount of light in the fiber;
FIG. 7 is a diagram corresponding to a product of a curve D and a curve D;

FIGS. 2A and 2B are diagrams for explaining the operation of another example of the optical transmission module according to the embodiment of the present invention, wherein FIG.
Is a schematic cross-sectional view of the optical transmission module, (b) is a diagram for explaining the operating characteristics of the optical transmission module, curve F is a graph showing the relationship between operating temperature and laser light output, curve G
Is a graph showing the relationship between operating temperature and optical coupling efficiency,
A curve H is a graph showing the relationship between the operating temperature, the laser light output, and the optical coupling efficiency, that is, a graph showing the amount of light in the fiber, and is a diagram corresponding to the product of the curve F and the curve G.

FIG. 3 is a diagram showing a result of measurement of a current amount of a semiconductor laser used as a light source of an optical transmission module according to an embodiment of the present invention, that is, a relationship between a driving current and an optical output, while changing a use temperature.

FIG. 4 is a diagram showing a relationship between a use temperature and a best image point position change amount when a resin condensing lens is used in another example of an optical transmission module according to an embodiment of the present invention. is there.

FIG. 5 is a schematic sectional view of a conventional optical transmission module.

FIG. 6 is a diagram for explaining a relationship between a critical incident angle θmax to an optical fiber and a total reflection angle θc.

[Explanation of symbols]

 Reference Signs List 21 semiconductor laser 22 condenser lens (aspherical condenser glass lens) 23 optical fiber fixing part 24 single mode optical fiber 25 case 26 optical fiber end face 51 semiconductor laser 52 condenser lens (resin condenser lens) 54 light Fiber 55 Case 56 Case 57 Optical fiber end face

Claims (1)

(57) [Claims] At least a semiconductor laser light source and said light source
For condensing light emitted from the device and transmitting the condensed light
Transmission module with optical fiber for
To the highest temperature in the operating temperature range of the optical transmission module.
The optical coupling efficiency to the optical fiber in
A light source; a means for condensing light emitted from the light source;
And an optical fiber for transmitting the illuminated light.
The amount of current injected into the semiconductor laser light source depends on the temperature.
Means for changing the optical coupling efficiency to the optical fiber in response to a change in the rise in operating temperature.
On the other hand, the light output of the light source
Characterized by the property of decreasing with respect to changes in ascent
Optical transmission module.