JP2013206974A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter which can achieve a desired optical output, while giving a drive current sufficient for obtaining an excellent relaxation oscillation frequency to a semiconductor laser, even in an environment where the temperature of the optical transmitter varies.SOLUTION: An optical transmitter 10 is a TOSA including a semiconductor laser diode 18, a variable optical attenuator 20, a fiber stub 28, a sleeve 30, and the like. The semiconductor laser diode 18 has such temperature characteristics that the optical output decreases as the temperature rises. The variable optical attenuator 20 is controlled so as to pass the laser light of the semiconductor laser diode 18 while attenuating, and to reduce the attenuation as the temperature of the semiconductor laser diode 18 rises. The fiber stub 28 and the sleeve 30 are coupled optically with an optical fiber when it is fixed, and have a role of fixing the optical fiber to a position where the laser light passed through the variable optical attenuator 20 enters.

Description

  The present invention relates to an optical transmitter.

  Conventionally, for example, as disclosed in Patent Document 1, an optical transmitter including an optical attenuator is known. 2. Description of the Related Art Currently widely used optical transmitters are equipped with a semiconductor laser inside and output an optical signal corresponding to an electrical signal using the semiconductor laser. The optical signal is transmitted through an optical waveguide member such as an optical fiber and used for optical communication.

  The semiconductor laser oscillates by being driven beyond a certain threshold current, and emits a stronger optical output as the drive current increases. The value of the drive current is also an element that determines the relaxation oscillation frequency, and the relaxation oscillation frequency is known as a characteristic related to the performance of the optical transmitter. This relaxation oscillation frequency is an element that determines the mask margin. Here, the mask margin (MM: MASK MARGIN) is an index for quantitatively evaluating the transmission waveform quality defined in international standards such as IEEE or ITU-T, and the transmission waveform conflicts with the mask. Means a margin up to. From the viewpoint of transmission waveform quality, it is preferable to obtain a good MM, and for this purpose, it is known that the relaxation oscillation frequency of the semiconductor laser needs to be increased.

  There are roughly two methods for increasing the relaxation oscillation frequency. The first method is to improve the characteristics of the semiconductor laser, specifically, to improve the differential gain. This first method is very difficult because it is necessary to greatly improve the characteristics of the semiconductor laser. On the other hand, as a second method, there is a method of adjusting driving conditions. This second method can be realized by increasing the light output and raising the operating point. This is because the relationship between the driving current and the relaxation oscillation frequency is that the relaxation oscillation frequency is proportional to the square root of the driving current.

  However, the intensity of the optical output actually output to the outside of the optical transmitter is limited. Such light output restrictions are required by, for example, international standards represented by ITU-T. In an international standard represented by ITU-T, for example, OC-192SR-1, the upper limit standard of optical output is strict, such as Pf = -6 to -1 dBm. If the operating point is simply raised, the light output may exceed the standard. As a technique for coping with this point, as disclosed in Patent Document 1, an optical attenuator is inserted between the semiconductor laser and the fiber stub to attenuate the laser light to a desired intensity, so that the light to the outside of the optical transmitter can be attenuated. There is something to adjust the output.

JP 2008-170636 A JP-A-2-230221 JP-A-4-290485

  The semiconductor laser has a temperature characteristic. When the driving current is the same, there is a characteristic that the light output decreases as the temperature of the semiconductor laser increases. In general, an operating temperature range having a certain range is defined for an optical transmitter. In practice, the optical transmitter is used at various temperature conditions within its operating temperature range, and is not always used at the same temperature. Therefore, it is assumed that the magnitude of the light output changes within a certain range according to the change in operating temperature.

  The optical attenuator described in Patent Document 1 is provided as a fixed optical attenuator having a constant optical attenuation of 3 dB or the like, as described in paragraphs 0012, 0019, and 0020 thereof or in FIG. Yes. Depending on the optical attenuator having such a single optical attenuation, it is difficult to cope with the semiconductor laser temperature characteristics as described above.

  In other words, because the semiconductor laser output is relatively high on the low temperature side according to the temperature characteristics, the optical attenuation of the optical attenuator should be set sufficiently large from the design philosophy to suppress the optical output to the outside of the optical transmitter. Can do. However, in this case, the sufficiently large light attenuation is uniformly applied even at the time of high temperature operation where the light output is reduced. As a result, since the optical output to the outside of the optical transmitter is too low on the high temperature side, the drive current must be increased. In particular, the drive current cannot always be increased without limit, and the supply current of the drive circuit (laser driver) usually has an upper limit. When the light attenuation amount is uniformly determined by ignoring such circumstances, the upper limit value of the light output is limited on the low temperature side and the supply current of the drive circuit is limited on the high temperature side, making it difficult to achieve both.

  As described above, the conventional technology does not take into consideration that the current-light output characteristics of the semiconductor laser have temperature characteristics. In this respect, there is still room for improvement.

  The present invention has been made to solve the above-described problems, and provides a semiconductor laser with a driving current sufficient to obtain a good relaxation oscillation frequency even in an environment where the temperature of an optical transmitter changes. An object of the present invention is to provide an optical transmitter capable of realizing a desired optical output.

The present invention is an optical transmitter comprising:
A semiconductor laser whose optical output decreases as the temperature rises;
A light attenuating unit that attenuates and passes the laser light of the semiconductor laser, and the amount of attenuation decreases as the temperature of the semiconductor laser increases;
A fixing portion for fixing the optical waveguide member at a position where the laser light having passed through the light attenuating portion is incident;
It is characterized by providing.

  According to the present invention, the amount of attenuation of laser light in the light attenuating portion can be reduced as the temperature of the semiconductor laser is higher. There is provided an optical transmitter capable of realizing a desired optical output while giving a driving current sufficient to obtain a good relaxation oscillation frequency to a semiconductor laser from a low temperature side to a high temperature side.

It is sectional drawing which shows the internal structure of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating operation | movement of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating operation | movement of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating operation | movement of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating operation | movement of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating operation | movement of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating the effect of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating the effect of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating the effect of the optical transmitter concerning Embodiment 1 of this invention. It is a figure for demonstrating the effect of the optical transmitter concerning Embodiment 1 of this invention. It is sectional drawing which shows the internal structure of the optical transmitter concerning Embodiment 2 of this invention. The light transmittance characteristic according to the wavelength of the optical filter concerning Embodiment 2 of this invention is shown. It is a figure which shows the temperature dependence curve of the differential efficiency which the semiconductor laser diode concerning Embodiment 2 of this invention has. It is sectional drawing which shows the internal structure of the optical transmitter concerning Embodiment 3 of this invention. It is a figure which shows the light transmittance characteristic according to the wavelength of the lens coating film concerning Embodiment 3 of this invention, and a stub coating film. It is a figure which shows the temperature dependence curve of the differential efficiency which a semiconductor laser diode has regarding Embodiment 3 of this invention. It is sectional drawing which shows the internal structure of the optical transmitter concerning Embodiment 4 of this invention. It is a figure which shows the light absorption characteristic in the electro-absorption layer implement | achieved by adjustment of the electric field by the EA control part in Embodiment 4 of this invention. It is a figure which shows the temperature dependence curve of the differential efficiency which a semiconductor laser diode has regarding Embodiment 4 of this invention. It is a figure which shows the modification of the optical transmitter concerning Embodiment 4 of this invention.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a cross-sectional view showing the internal structure of the optical transmitter 10 according to the first embodiment of the present invention. The optical transmitter 10 is a TOSA (Transmitter Optical Sub Assembly), that is, a small optical device module for transmission. In the present embodiment, the optical transmitter 10 has a use temperature range of minus 5 ° C. as the low temperature side use lower limit temperature and 85 ° C. as the high temperature side use upper limit temperature.

  The optical transmitter 10 includes a stem 12. A metal block 14 is provided on the stem 12, and a semiconductor laser diode 18 and a variable optical attenuator 20 are attached on the metal block 14 via a submount 16. The semiconductor laser diode 18 has a light output that decreases as the temperature rises. Specifically, the semiconductor laser diode 18 is an uncooled DFB laser diode (Uncooled DFB-LD). The variable optical attenuator 20 can attenuate and pass the laser light of the semiconductor laser diode 18.

  A temperature monitor element 21 is attached to the metal block 14, and the temperature of the semiconductor laser diode 18 can be detected by the output of the temperature monitor element 21. The stem 12 is provided with pins 13 (pins 13a, 13b, 13c, 13d) which are lead pins. Each pin is a semiconductor laser diode 18, variable optical attenuation by a metal wire (not shown) as a signal line. Connected to the vessel 20 and the temperature monitoring element 21.

  A lens cap 22 including a condenser lens 24 is fixed to the stem 12 by electric welding. The optical transmitter 10 has a so-called CAN package structure. The condenser lens 24 is provided between the variable optical attenuator 20 and an optical fiber (optical waveguide member) (not shown), and the light that has passed through the variable optical attenuator 20 is coupled to collect the collected light. Transmitted to the optical waveguide member). The internal space covered with the lens cap 22 is hermetically sealed.

  Laser light from the semiconductor laser diode 18 reaches the condenser lens 24 via the variable optical attenuator 20. A fiber stub 28 and a sleep 30 are further connected to the cap 22 with a lens, and an optical isolator 26 is provided between the fiber stub 28 and the condenser lens 24. The sleep 30 can fix the optical fiber (optical waveguide member) at a position where the laser light that has passed through the light attenuating portion is incident. The optical isolator 26 is provided between the condenser lens 24 and an optical fiber (optical waveguide member) (not shown), and cuts return light from the optical fiber (optical waveguide member) side (not shown).

  The variable optical attenuator 20 is also referred to as a VOA (Variable Optical Attenuator). The variable optical attenuator 20 is a MEMS (Micro Electro Mechanical Systems) type variable optical attenuator. MEMS optical attenuators are generally small and easy to mount on TOSA.

  The variable optical attenuator 20 is connected to the control unit 32. The control unit 32 includes a drive circuit 32 a that is a laser driver that drives the semiconductor laser diode 18, and a control circuit 32 b that controls the amount of optical attenuation of the variable optical attenuator 20. The control unit 32 may control the variable optical attenuator 20 so as to decrease the attenuation amount of the variable optical attenuator 20 in accordance with an increase in the temperature of the semiconductor laser diode 18 based on the output signal of the temperature monitor element 21. it can. The controller 32 can independently control the drive current of the semiconductor laser diode 18 and the optical attenuation amount of the variable optical attenuator 20 (that is, the optical output of the optical transmitter 10).

  The drive circuit 32a can execute APC control for supplying a drive current to the semiconductor laser diode 18 within a range equal to or lower than the upper limit current value and controlling the drive current of the drive circuit so that the optical output becomes a predetermined output value. Although a monitor light-receiving element (photodiode) necessary for APC control is not shown, it may be used by attaching it to a position where the output of the optical transmitter 10 can be detected as appropriate. Further, the change in the optical output of the semiconductor laser diode 18 due to the temperature change can be detected by the light receiving element for monitoring. Therefore, the temperature monitoring element 21 may be omitted.

[Operation of Embodiment 1]
2 to 6 are diagrams for explaining the operation of the optical transmitter 10 according to the first embodiment of the present invention.

(Relationship between drive current, light output and relaxation oscillation frequency)
2 and 3 are diagrams showing the relationship between the drive current, the optical output, and the relaxation oscillation frequency.
As shown in the current-light output characteristic 40, there is a linear relationship between the drive current and the light output. Further, as shown in the relaxation oscillation frequency characteristic 42, there is a relationship that the relaxation oscillation frequency is proportional to the square root of the drive current between the drive current and the relaxation oscillation frequency. These relationships are shown in FIG. As the drive current increases, the optical output increases, and the relaxation oscillation frequency also increases in proportion to the square root of the drive current.

As shown in FIG. 2, the light output, the light output upper limit value P _L which is determined by the standards or the like are present. This _{P L,} a current value _{I PL} is determined. The I _PL is the current value Ritsusa in light output upper limit value. On the other hand, there is a relaxation vibration frequency f _ri necessary for obtaining a good mask margin (MM) with respect to the relaxation vibration frequency. Also from this f _ri , one current value I _i is determined. The current value I _i is an ideal drive current value from the viewpoint of the relaxation oscillation frequency.

FIG. 3 is a diagram for explaining an effect achievable when the optical transmitter 10 according to the first embodiment of the present invention includes the variable optical attenuator 20. By interposing the optical attenuator, the slope of the light output with respect to the current value (current light output characteristics) can be made gentle. As shown in FIG. 3, the current light output characteristic 40 can be adjusted from the current light output characteristic 48 as shown by the arrow in FIG. 3, and the coupling efficiency can be adjusted by the variable optical attenuator. By adjusting to an appropriate coupling efficiency using the variable optical attenuator 20 so as not to exceed the optical output upper limit value _IPL at the ideal driving current I _i , a good mask margin can be obtained.

(Optimization of optical attenuation for temperature characteristics of semiconductor lasers)
In general, a semiconductor laser has temperature dependence with respect to the relationship between drive current and optical output. As shown in FIG. 4, when the current-light output characteristic TL1 at low temperature and the current-light output characteristic TR1 at room temperature are compared, the slope of the current-light output characteristic TL1 at low temperature is steeper. That is, differential efficiency increases at room temperature compared to room temperature, so that the optical output becomes relatively higher than room temperature even when driven with the same current. In that case, the limit of the drive current I _PL by the upper limit value P _L of the light output becomes more severe than the room temperature. That is, if the drive current is increased at a low temperature, the light output becomes too large, so the drive current has to be reduced compared to room temperature. However, on the other hand, if the drive current is small, the relaxation oscillation frequency must be lowered.

Therefore, light is attenuated so that the current-light output characteristic TL2 is changed from the current-light output characteristic TL1 at low temperature. In this way, by reducing the coupling efficiency corresponding to the increment of the light output, while maintaining a high relaxation oscillation frequency (while maintaining above f _ri), the light output equal to or less than the upper limit value P _L of the optical output Can be controlled.

  Optical attenuation has the effect of making the slope of the current-light output characteristic gentle (the effect of changing from TL1 to TL2 in FIG. 4). According to the variable optical attenuator 20, the amount of optical attenuation can be changed. Accordingly, it is possible to vary the degree of gently changing the slope of the current-light output characteristics. At low temperatures, the amount of light attenuation can be relatively increased to increase the degree of change in the inclination. In FIG. 4, an arrow Att1 from TL1 to TL2 indicates this.

  Conversely, at high temperatures, the amount of light attenuation can be made relatively small, and the degree of change in the tilt can be made small. In FIG. 4, an arrow Att2 from the current light output characteristic TR1 to the current light output characteristic TR2 at room temperature indicates this. Comparing the arrow Att1 and the arrow Att2, the arrow Att1 is relatively longer, indicating that the light attenuation is larger.

  The effect of the variable optical attenuator 20 will be further described with reference to FIGS. FIG. 5 is a diagram showing current-light output characteristics when the coupling efficiency is uniformly reduced using an optical attenuator having a constant optical attenuation. Current light output characteristics TL3 and TH1 in FIG. 5 are current light output characteristics of the semiconductor laser diode 18 at a low temperature and a high temperature, respectively. On the other hand, the current-light output characteristics TL4 and TH2 are current-light output characteristics for an optical signal output to the outside of the optical transmitter via the optical attenuator at low temperatures and high temperatures, respectively. In addition, the low temperature and high temperature here mean the difference in relative temperature.

When an optical attenuator with a constant light attenuation is used, not only does it have an attenuation effect on the current-light output characteristics on the low-temperature side, but also the high-temperature current-light output characteristics are comparable to this. The rate becomes moderate. Then, in order to obtain the same light output on the high temperature side as compared with the low temperature side, it is necessary to further increase the drive current, and it is necessary to increase the drive current unintentionally by the optical attenuator. This inevitably becomes a problem when driving the optical transmitter to keep the optical output constant, that is, when performing APC (Auto Power Control). This is because when APC driving is performed, a high temperature driving current must be increased in order to secure the target light output _PAPC .

However, on the other hand, the drive circuit has an upper limit value I _CL of the current, and it is normal that current cannot be supplied beyond a certain fixed value. That is, a drive current can be applied to the semiconductor laser diode 18 only within a range equal to or less than _ICL . Therefore, it is necessary to obtain a desired light output (the target value P _{APC in the} case of APC driving) below the supply current upper limit value I _{CL of} the driving circuit. As in Figure 5, in order to obtain a P _APC at a current optical output characteristics TH2 at a high temperature, the forced supply I _TH2IPC a "hot drive current when the coupling efficiency is lowered uniformly" . The _{I TH2IPC} has exceeded _{I CL,} can not be supplied in a driving circuit for a current upper limit value _{I CL.}

  FIG. 6 shows a case where the variable optical attenuator 20 is applied. As described above, since the variable optical attenuator 20 can change the optical attenuation, the optical attenuation is sufficiently increased and the coupling efficiency is adjusted in order to maximize the relaxation oscillation frequency on the low temperature side. . As a result, the laser light output of the semiconductor laser diode 18 can be adjusted (attenuated) from the current light output characteristic TL5 at the low temperature of FIG. 6 to the current light output characteristic TL6.

On the other hand, on the high temperature side, the optical attenuation is adjusted to a small value so that the drive current does not exceed the supply current upper limit value, that is, _ICL . As a result, the laser light output of the semiconductor laser diode 18 can be adjusted (attenuated) from the current light output characteristic TH3 at the time of high temperature in FIG. 6 to the current light output characteristic TH4. As a result, the current-light output characteristics after attenuation (TL6, TH4) become characteristics that can obtain the light output P _APC with respect to the drive current I _APC at both low temperature and high temperature. In other words, I _APC is “a driving current at a high temperature when the variable optical attenuator 20 sets the optimum coupling efficiency depending on the temperature”.

(Contents of preferred control executed by the control unit 32)
Hereinafter, the contents of preferable control executed by the control unit 32 in the optical transmitter 10 according to the first embodiment of the present invention will be described. As preferable control, the first control content and the second control content will be described below.

The first control content is as follows.
The attenuation rate of the variable optical attenuator 20 is set to a small value, for example, 0%, at the upper limit (for example, 85 ° C.) of the operating temperature range. In this state, the drive current of the semiconductor laser diode 18 is set so that a predetermined optical output _PAPC is obtained. The drive current is set to a value equal to or less than the upper limit value I _CL of the supply current of the drive circuit 32a.

When the light output of the semiconductor laser diode 18 is small and a necessary relaxation frequency cannot be obtained due to temperature change, the drive current of the semiconductor laser diode 18 is increased and the attenuation factor of the variable optical attenuator 20 is increased. Thus, while maintaining the optical output of the optical transmitter 10 to a predetermined value P _APC, so that the necessary relaxation frequency f _ri is obtained.

At other temperatures (room temperature, low temperature), the drive current of the semiconductor laser diode 18 is made constant and the attenuation rate of the variable optical attenuator 20 is changed. Increasing the decay rate with decreasing temperature. The control unit 32 suppresses a change in the intensity of incident light on the optical fiber (not shown) when the drive current of the semiconductor laser diode 18 is held at a predetermined value (a predetermined value equal to or less than _ICL ). The amount of attenuation of the variable optical attenuator 20 is changed. Thereby, the optical transmitter 10 is APC driven.

The second control content is as follows. In the above first control content, the driving current of the semiconductor laser diode 18 is made constant. In the second control content, the attenuation rate of the variable optical attenuator 20 is made constant and the drive current is adjusted in the temperature range where the necessary relaxation oscillation frequency f _ri can be obtained. Usually, the current is decreased with decreasing temperature.

In other temperature ranges (temperature range in which the relaxation oscillation frequency f _ri cannot be obtained), the optical module is driven by APC by changing the attenuation rate of the variable optical attenuator 20 while keeping the drive current of the semiconductor laser diode 18 constant. To do. That is, the control unit 32 adjusts the variable light so as to suppress the change in the intensity of the incident light to the optical fiber (not shown) when the drive current is held at a predetermined value (a predetermined value equal to or less than I _CL ). The amount of attenuation of the attenuator 20 is changed. Thereby, the optical transmitter 10 is APC driven.

  According to the optical transmitter 10 according to the first embodiment described above, the amount of optical attenuation in the variable optical attenuator 20 can be reduced as the temperature of the semiconductor laser diode 18 increases. As a result, the light attenuation amount is relatively increased on the low temperature side where the output of the semiconductor laser diode 18 is relatively large due to the temperature characteristics with respect to a certain drive current, and the output of the semiconductor laser diode 18 is relatively increased due to the temperature characteristics. On the small high temperature side, the light attenuation can be relatively increased.

In this way, a desired light output (P _L or less) is given to the semiconductor laser diode 18 with a drive current I _i sufficient to obtain a good relaxation oscillation frequency (f _ri ) from the low temperature side to the high temperature side. An optical transmitter 10 capable of realizing a predetermined output or P _APC ) is provided. Thereby, the optical transmitter 10 can obtain a good mask margin in a wide temperature range.

The upper limit of the attenuation amount of the variable optical attenuator 20 is that the optical output of the semiconductor laser diode 18 is changed to the optical output upper limit value P _L of the optical transmitter 10 when the optical transmitter 10 is at the lower limit operating temperature (minus 5 ° C.). It is greater than or equal to the following attenuation. Further, the lower limit of the attenuation amount of the variable optical attenuator 20 is the driving state in which the semiconductor laser diode 18 is driven with the upper limit value I _CL of the driving current when the optical transmitter 10 is at the upper limit use temperature (85 ° C.). The optical output of the optical transmitter 10 in the driving state is equal to or less than an attenuation amount at which a predetermined output value (P _APC or P _L ) is obtained. As a result, the amount of light attenuation can be adjusted so as to realize the ideal relaxation oscillation frequency f _ri in the temperature range between the lower limit operating temperature and the upper limit operating temperature.

(Evaluation of eye pattern, etc.)
7 to 10 are diagrams for explaining the effect of the optical transmitter 10 according to the first embodiment of the present invention. FIG. 7 shows the temperature dependence of the relaxation oscillation frequency during APC driving. Since the relaxation oscillation frequency is also proportional to the square root of the differential gain, and the differential gain is determined by the detuning amount, a curve as shown in FIG. 7 is generally drawn. By optimizing the coupling efficiency for each temperature by the variable optical attenuator 20, the relaxation oscillation frequency can be increased in a wide temperature range, and the relaxation oscillation frequency characteristic can be improved from the characteristic 52 to the characteristic 50. As a result, a good mask margin can be obtained.

  FIG. 8 shows the characteristics of the relaxation oscillation frequency (fr) when the fourth-order Bessem Thomson filter is not used, and FIG. 9 shows the eye when the fourth-order Bessem Thomson filter is used when the relaxation oscillation frequency is low. FIG. 10 shows a good eye pattern when the fourth-order Bessem Thomson filter is used when the relaxation oscillation frequency is high. According to the present embodiment, good characteristics as shown in FIG. 10 can be obtained, and a good mask margin can be obtained.

  In the first embodiment, an uncooled DFB laser diode is used as the semiconductor laser diode 18. However, the present invention is not limited to this. An edge emitting type (Fabry-Perot type) semiconductor laser or a surface emitting type semiconductor laser may be used. Moreover, although the optical transmitter 10 has a CAN package structure, the present invention is not limited to this, and may be a butterfly package or other various packages. Further, the optical transmitter according to the present invention is not necessarily provided as TOSA.

Embodiment 2. FIG.
FIG. 11 is a cross-sectional view showing an internal configuration of the optical transmitter 100 according to the second embodiment of the present invention. The optical transmitter 100 is different from the optical transmitter 10 in that it does not have the variable optical attenuator 20 but has an optical filter 120 instead. Further, the control unit 32 is not provided, and instead, a drive circuit 132 for driving the semiconductor laser diode 18 is provided. Accordingly, the number of pins 13 is also reduced. Since other configurations are the same as those of the optical transmitter 10, the same reference numerals are given and description thereof is omitted.

  The semiconductor laser diode 18 has an oscillation peak wavelength that changes to the longer wavelength side as the temperature rises. That is, as a characteristic of the uncooled DFB laser diode, the oscillation peak wavelength has temperature dependence, and the oscillation peak wavelength changes depending on the temperature from the low temperature side to the short wavelength side and from the high temperature side to the long wavelength side. The semiconductor laser diode 18 is assumed to have a temperature dependency of approximately 0.1 nm / ° C.

  The optical transmitter 100 includes an optical filter 120 interposed between the condenser lens 24 and the fiber stub 28. More specifically, the optical filter 120 arranged in such a positional relationship is provided between the optical isolator 26 and the fiber stub 28. Further, it is interposed between the semiconductor laser diode 18 and the fiber stub 28 when viewed in the optical axis direction.

  The optical filter 120 is a light transmission layer whose attenuation changes so that the transmittance becomes higher as the wavelength of the incident laser light is longer. FIG. 12 shows light transmittance characteristics according to the wavelength of the optical filter 120 according to the second embodiment of the present invention. As shown in FIG. 12, the optical filter 120 suppresses the light transmittance on the short wavelength side and increases the transmittance on the long wavelength side. In this way, the amount of light transmitted through the optical filter 120 can be increased (the amount of light attenuation can be reduced) as the oscillation peak wavelength shifts to the longer wavelength side as the temperature of the semiconductor laser diode 18 rises. ).

  The best effect can be obtained when the transmittance curve of the optical filter 120 is designed to cancel the temperature dependence of the differential efficiency of the semiconductor laser diode 18. That is, the best effect is obtained when the temperature dependence curve of the differential efficiency of the semiconductor laser diode 18 as shown in FIG. 13 and the transmittance characteristic curve shown in FIG. It is done.

Embodiment 3 FIG.
FIG. 14: is sectional drawing which shows the internal structure of the optical transmitter 200 concerning Embodiment 3 of this invention. The optical transmitter 200 is different from the optical transmitter 10 in that it does not have the variable optical attenuator 20 but has a lens coating film 220 and a stub coating film 228 instead. Further, the control unit 32 is not provided, and instead, a drive circuit 132 for driving the semiconductor laser diode 18 is provided. Accordingly, the number of pins 13 is also reduced. Since other configurations are the same as those of the optical transmitter 10, the same reference numerals are given and description thereof is omitted.

  Also in the third embodiment, as in the second embodiment, the semiconductor laser diode 18 has an oscillation peak wavelength that changes to the longer wavelength side as the temperature rises. That is, as a characteristic of the uncooled DFB laser diode, the oscillation peak wavelength has temperature dependence, and the oscillation peak wavelength changes depending on the temperature from the low temperature side to the short wavelength side and from the high temperature side to the long wavelength side. The semiconductor laser diode 18 is assumed to have a temperature dependency of approximately 0.1 nm / ° C.

  The optical transmitter 100 includes a condenser lens 24, an optical isolator 26, and a fiber stub 28 as optical components interposed between the semiconductor laser diode 18 and an optical fiber (optical waveguide member) to be optically coupled thereto. Yes. In the optical transmitter 100, among these optical components, a lens coating film 220 is provided on the surface of the condensing lens 24 (at least a portion to which the laser beam is coupled), and the stub is formed on the tip surface (light incident end surface) of the fiber stub 28. A coating film 228 is provided.

  The lens coating film 220 and the stub coating film 228 are light transmission layers whose transmittance increases as the wavelength of incident laser light is longer. That is, the amount of optical attenuation decreases as the wavelength of the incident laser beam is longer.

  FIG. 15 shows light transmittance characteristics according to the wavelengths of the lens coating film 220 and the stub coating film 228 according to the third embodiment of the present invention. As shown in FIG. 15, the lens coating film 220 and the stub coating film 228 suppress the light transmittance on the short wavelength side and increase the transmittance on the long wavelength side. By doing so, the amount of light transmitted through the lens coating film 220 and the stub coating film 228 can be increased as the oscillation peak wavelength shifts to the longer wavelength side as the temperature of the semiconductor laser diode 18 rises ( Light attenuation can be reduced).

  As in the case of the optical filter 120 in the second embodiment, when the transmittance curves of the lens coating film 220 and the stub coating film 228 are designed to cancel the temperature dependence of the differential efficiency of the semiconductor laser diode 18. In addition, the best effect can be obtained. That is, the best effect is obtained when the temperature dependence curve of the differential efficiency of the semiconductor laser diode 18 as shown in FIG. 16 and the transmittance characteristic curve shown in FIG. It is done.

Embodiment 4 FIG.
FIG. 17 is a cross-sectional view showing an internal configuration of the optical transmitter 200 according to the fourth embodiment of the present invention. The optical transmitter 200 does not include the variable optical attenuator 20, but instead includes a laser diode chip in which the semiconductor laser diode 318 and the electroabsorption layer 320 are integrated. It is different. Further, a control unit 332 is provided instead of the control unit 32. Since other configurations are the same as those of the optical transmitter 10, the same reference numerals are given and description thereof is omitted.

  The optical transmitter 300 includes a semiconductor laser diode 318 that is an uncooled DFB laser diode similar to the semiconductor laser diode 18. An electroabsorption layer 320 is integrated on the front end face side of the semiconductor laser diode 318. The electroabsorption layer 320 is a front end face side electroabsorption layer of the semiconductor laser diode 318. Such a configuration is a configuration known as a so-called electroabsorption optical modulator, and is also a configuration called an EA (Electro Absorption) modulator.

  The optical transmitter 300 includes a control unit 332. The control unit 332 includes a drive circuit 332 a as a laser driver that supplies a drive current to the semiconductor laser diode 318, and an EA control unit 332 b that adjusts the electric field applied to the electric field absorption layer 320. The EA control unit 332b adjusts the electric field applied to the electroabsorption layer 320 so that the light absorption amount of the electroabsorption layer 320 decreases as the temperature of the semiconductor laser diode 318 increases.

  In the optical transmitter 300 according to the fourth embodiment, the control unit 332 performs the following control. That is, a modulation operation (direct modulation) is performed on the semiconductor laser diode 318 by adjusting the drive current, and the electric field absorption layer 320 is operated as a variable optical attenuator by adjusting the electric field. Thus, the electric field of the electroabsorption layer 320 is adjusted according to the temperature, and a desired amount of light absorption (that is, a desired amount of light attenuation) is obtained.

Specifically, the EA control unit 332b applies a first electric field to the electroabsorption layer 320 so that the first light absorption amount is obtained when the semiconductor laser diode 318 is at a low temperature, and the semiconductor laser diode 318 is at a high temperature. For example, the second electric field is applied to the electroabsorption layer 320 so that the second light absorption amount is smaller than the first light absorption amount.
This is in contrast to an ordinary EA modulator that obtains modulated light by modulating the electroabsorption layer while performing CW (Continuous Wave Oscillation) driving for continuously transmitting the semiconductor laser diode portion.

  FIG. 18 is a diagram illustrating light absorption characteristics in the electroabsorption layer 320 realized by adjusting the electric field by the EA control unit 332b in the fourth embodiment. As the temperature of the semiconductor laser diode 318 rises, the light absorption rate is lowered. Accordingly, the higher the temperature of the semiconductor laser diode 318, the lower the light attenuation in the electroabsorption layer 320.

  As described above, the transmittance curve of the optical filter 120 in the second embodiment and the transmittance curve of the lens coating film 220 in the third embodiment cancel the temperature dependence of the differential efficiency of the semiconductor laser diode 18. It is preferable to be designed as follows. Similarly, the light absorption amount control of the electroabsorption layer 320 according to the temperature of the semiconductor laser diode 318 is also designed so that the temperature light absorption characteristic cancels the temperature dependence of the differential efficiency of the semiconductor laser diode 318. Is preferred. That is, the best effect is obtained when the temperature dependence curve of the differential efficiency of the semiconductor laser diode 318 shown in FIG. 19 and the light absorption characteristic curve shown in FIG. It is done.

  FIG. 20 is a diagram of a modification of the optical transmitter according to the fourth embodiment of the present invention. FIG. 20 is a partially enlarged view of the vicinity of the metal block 14 of the optical transmitter 300, and the configuration other than those illustrated individually is the same as that of FIG.

  This modified example includes a semiconductor laser diode chip 418 instead of the laser diode chip including the semiconductor laser diode 318 and the electroabsorption layer 320. The semiconductor laser diode chip 418 includes a DFB laser diode unit 419, a light output adjustment layer 420, and a monitor current extraction layer 422, which are connected to the control unit 432.

  The optical output adjustment layer 420 is an electroabsorption layer (modulator part in the EA modulator) provided on the front end face side, and the monitor current extraction layer 422 is an electroabsorption layer (modulation in the EA modulator) provided on the rear end face side. Instrument part). The DFB laser diode unit 419 has a configuration of an uncooled DFB laser diode similar to the semiconductor laser diode 318. The light output adjustment layer 420 corresponds to the electric field absorption layer 320. The optical transmitter according to the modification of the present embodiment illustrated in FIG. 20 includes a monitor current extraction layer 422. The monitor current extraction layer 422 is a rear end face side electric field absorption layer (or modulator section) integrated on the rear end face side of the semiconductor laser diode chip 418 and through which a current flows according to light absorption.

  The modification shown in FIG. 20 includes a control unit 432. The control unit 432 includes a feedback unit 432c in addition to the drive circuit 332a and the EA control unit 332b described above. The monitor current extraction layer 422 generates a photocurrent that is generated when light is absorbed. The value of this photocurrent is input to the feedback unit 432 c in the control unit 432. The feedback unit 432 c feeds back the value of the photocurrent flowing through the monitor current extraction layer 422 to the value of applied electrolysis of the light output adjustment layer 420. According to the present embodiment, the absorptance of the light output adjustment layer 420 and the monitor current value of the monitor current extraction layer 422 can be linked. This enables APC driving.

  As a technique for monitoring the optical output of the front end face of the semiconductor laser diode with reference to the monitor current, for example, there is a technique disclosed in Japanese Patent Laid-Open No. 4-290485. However, since the technique of the publication is a configuration in which the semiconductor laser diode portion, the monitor portion, and the modulator (light absorption layer) are arranged in this order, the configuration is different from the present embodiment. In this case, since the monitor current is not constant, APC driving with reference to the monitor current cannot be performed. In this regard, according to the fourth embodiment, APC driving can be realized by the control of the feedback unit 432c described above.

10, 100, 200, 300 Optical transmitter 12 Stem 13, 13a, 13b, 13c, 13d Pin 14 Metal block 16 Submount 18, 318 Semiconductor laser diode 20 Variable optical attenuator 21 Temperature monitor element 22 Cap with lens 24 Condensing Lens 26 Optical isolator 28 Fiber stub 30 Sleep 32 Control unit 32a Drive circuit 32b Control circuit 120 Optical filter 132 Drive circuit 220 Lens coating film 228 Stub coating film 320 Electric field absorption layer 332 Control unit 332a Drive circuit 332b Control unit 418 Semiconductor laser diode chip 419 Laser diode unit 420 Light output adjustment layer 422 Monitor current extraction layer 432 Control unit 432c Feedback unit

Claims (10)

A semiconductor laser whose optical output decreases as the temperature rises;
A light attenuating unit that attenuates and passes the laser light of the semiconductor laser, and the amount of attenuation decreases as the temperature of the semiconductor laser increases;
A fixing portion for fixing the optical waveguide member at a position where the laser light having passed through the light attenuating portion is incident;
An optical transmitter comprising: The light attenuator is
A variable optical attenuator that can change the amount of light attenuation;
A control unit for controlling the variable optical attenuator so as to reduce the attenuation amount of the variable optical attenuator according to an increase in temperature of the semiconductor laser;
The optical transmitter according to claim 1, comprising:   The control unit changes the attenuation amount of the variable optical attenuator so as to suppress a change in the intensity of incident light to the optical waveguide member when the driving current of the semiconductor laser is held at a predetermined value. The optical transmitter according to claim 2, wherein the optical transmitter is one. A condensing lens provided between the variable optical attenuator and the optical waveguide member, the light passing through the variable optical attenuator is coupled, and the condensed light is transmitted to the optical waveguide member;
An optical isolator that is provided between the condenser lens and the optical waveguide member and cuts the return light from the optical waveguide member side;
The optical transmitter according to claim 2, further comprising:   The upper limit of the attenuation amount of the variable optical attenuator is equal to or greater than an attenuation amount that makes the optical output of the semiconductor laser not more than the upper limit value of the optical output of the optical transmitter when the optical transmitter is at the lower limit operating temperature. The optical transmitter according to any one of claims 2 to 4. A drive circuit capable of performing APC control for controlling the drive current so as to give a drive current to the semiconductor laser in a range equal to or lower than an upper limit current value and to set the optical output to a predetermined output value;
The lower limit of the attenuation amount of the variable optical attenuator is that the optical output of the optical transmitter in the driving state is the driving state in which the semiconductor laser is driven at the upper limit current value when the optical transmitter is at the upper limit operating temperature. The optical transmitter according to claim 2, wherein the optical transmitter is equal to or less than an attenuation amount at which the predetermined output value is obtained. The semiconductor laser has an oscillation peak wavelength that changes on the long wavelength side in response to a temperature rise,
2. The optical transmitter according to claim 1, wherein the light attenuating unit is a light transmitting layer having a higher transmittance as the wavelength of incident laser light is longer. The light transmission layer is
An optical filter provided at a position between the semiconductor laser and the optical waveguide member;
A coating film provided on the surface of an optical component disposed at a position between the semiconductor laser and the optical waveguide member;
The optical transmitter according to claim 7, wherein the optical transmitter is at least one of the two. The light attenuator is
A front end face side electroabsorption layer integrated on the front end face side of the semiconductor laser;
An electric field adjusting unit that adjusts an electric field applied to the front end surface side electroabsorption layer so that the amount of light absorption of the front end surface side electroabsorption layer decreases as the temperature of the semiconductor laser increases;
The optical transmitter according to claim 1, comprising: A rear end face side electric absorption layer that is integrated on the rear end face side of the semiconductor laser, and a current flows according to light absorption;
A feedback adjustment unit that adjusts the electric field applied to the front end surface side electroabsorption layer so as to interlock the value of current flowing in the rear end surface side electroabsorption layer and the light absorption amount of the front end surface side electroabsorption layer;
The optical transmitter according to claim 9, further comprising:

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