Abstract:
The present invention is to provide an optical module with a co-axial package and to enable to measure the temperature of the semiconductor laser diode in accurate. The module of the present invention includes the primary portion and the package. The package has the stem and the cap mounted on the stem and covering the primary portion. The primary portion includes the thermoelectric element mounted on the stem, the sub-mount installed onto the thermoelectric element, the laser diode mounted on the sub-mount, and the resin as the shielding member fixed to the sub-mount and to thermally shield the thermistor from the cap by covering the thermistor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical module, in particular, the invention relates to the optical module with a thermo-electric element within a co-axial package.  
         [0003]     2. Related Prior Arts  
         [0004]     Two shapes of the package have been well known for the optical transmitting module installing a semiconductor laser diode. One is what is called a butterfly package with a box shape, which has disclosed in the Japanese Patent published as JP-2003-142767A. The other is what we call a co-axial package with a cylindrical shape, which has disclosed in the Japanese Patent published as JP-2003-142766A.  
         [0005]     The former module includes a butterfly package with the box shape, a base installed within the package, a thermo-electric element arranged between the base and the package, a semiconductor laser diode (hereinafter denoted as LD) mounted on the thermo-electric element as an element to be cooled down, a lens, and a photodiode (hereinafter denoted as PD). In this module, by covering the devices mounted on the base with a thermal sheet, heat conducted from the package to the devices by the radiation may be cut, which reduces the thermal load of the thermo-electric element, thereby decreasing the power consumption.  
         [0006]     On the other hand, the module disclosed in the latter prior art includes the thermo-electric element, the LD, and the thermistor. The LD and the thermistor are mounted on the thermo-electric element via the carrier. The LD may be controlled in the temperature thereof by the thermo-electric element within such small-sized co-axial package.  
         [0007]     However, the module with the co-axial package, which has an advantage that the package thereof is smaller than the butterfly package, has the following subjects due to its small sized package. That is, because a distance between the thermistor and the wall of the case becomes close, the thermistor is easy to be influenced from the temperature of the case, namely, the ambient temperature of the module. Specifically, when the temperature of the LD is set to T [° C.], the thermo-electric element may be overcooled, or overheated, to the temperature (T−Δ) [° C.] because the signal output from the thermistor reflects the increase or decrease in the temperature by Δ[° C.] due to the radiation from the case. Accordingly, the LD is hard to be accurately controlled to a specific temperature.  
         [0008]     When such module is applied to the dense wavelength division multiplexing (DWDM) system, this subject will be fatal. The emission wavelength of the LD has a temperature dependence, while, the variation of the emission wavelength of the LD is necessary to be suppressed because the span of the signal wavelengths in the DWDM system is set quite narrow. The conventional module suppresses the variation of the emission wavelength by setting a control circuit that maintains the emission wavelength of the LD constant in the outside of the module. However, such system enlarges the circuit size.  
         [0009]     Moreover, the thermal sheet such as those used in the former document, which covers the LD together with other devices, is hard to be installed within the co-axial package. Since the co-axial package holds the lens with the case thereof, the distance between the LD and the inner surface of the case is quite small, which becomes hard to install the thermal sheet.  
         [0010]     The present invention is to provide a module with a co-axial package and capable of precisely controlling thin temperature of the  
       SUMMARY OF THE INVENTION  
       [0011]     An optical module according to the present invention has a characteristic to provide a co-axial package, a semiconductor laser diode installed in the co-axial package, and a thermoelectric element to control the operating temperature of the laser diode. The package includes a stem and a cap fixed to the stem. The thermoelectric element is mounted on the stem, and the semiconductor laser diode is mounted on the stem. Moreover, a temperature sensor, for instance, a thermistor to monitor the temperature of the semiconductor laser diode, namely, the temperature on the thermoelectric element, is also mounted on the thermoelectric element. The first embodiment according to the present invention is that the temperature sensor is covered by the shielding member so as to be thermally isolated from the cap.  
         [0012]     Since the temperature sensor is thermally isolated from the cap, the temperature sensor can monitor the temperature on the thermoelectric element, namely, the temperature of the semiconductor laser diode indirectly, as reducing the influence from the ambient temperature, which enables to precisely control the thermoelectric element in precise and reduces the drift of the emission wavelength of the semiconductor laser diode against the ambient temperature.  
         [0013]     The shielding member may be an epoxy resin containing silicon dioxide (Si 0   2 ) or aluminum nitride (AlN), and may cover the temperature sensor (thermistor) on the top of the thermoelectric element. Or, the shielding member may be a slab made of metal or ceramics fixed to the thermoelectric element and interposed between the temperature sensor and the cap. The temperature sensor, or the semiconductor laser diode, mounted on the thermoelectric element via a carrier, accordingly, the resin or the slab as the shielding member may be fixed to the carrier. Moreover, the slab may be integrally formed with the carrier. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  is a partially broken perspective view of the optical module according to the first embodiment of the invention;  
         [0015]      FIG. 2  is a cross section taken along the ling II-II in  FIG. 1 ;  
         [0016]      FIG. 3A  shows a shirt of the emission wavelength of the LD, namely, the wavelength drift, against the temperature of the LD of the optical module according to the embodiment, and  FIG. 3B  shows the wavelength drift without the resin;  
         [0017]      FIG. 4  is a partially broken perspective view of the optical module according to the second embodiment of the present invention; and  
         [0018]      FIG. 5  is a cross section of the optical module shown in  FIG. 4  taken along the line V-V.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]     Next, embodiments according to the present invention will be described in detail as referring to accompanying drawings. In the description of the drawings, same elements will be referred by the same numeral without overlapping explanations.  
       First Embodiment  
       [0020]      FIG. 1  is a partially broken perspective view of a module  1   a  according to the first embodiment of the invention.  FIG. 2  is a cross section of the module  1   a  shown in  FIG. 1  taken along the line IIT. The module  1   a  provides a primary portion  20   a  and a package  30 . Here, the module  1   a  of the present embodiment has, what is called, a co-axial package.  
         [0021]     The package  30  has a stem  31  and a cap  32 . The stem  31  includes a plurality of lead pins  31   a  and a base  31   b  holding these lead pins  31   a.  The lead pin  31   a  extends along the axis X and, in  FIG. 1 , four lead pins are collectively arranged to pass the base  31   b.  The base  31   b  provides a surface  31   c  intersecting the axis X. On the surface  31   a  is mounted with the primary portion  20   a  that will be described later.  
         [0022]     The cap  32  has a lens cap  32   a  with a cylindrical shape and a lens  32   c  fitted within an opening  32   b  of the lens cap  32   a . The lens cap  32   a  covers the primary portion  20   a  and fixed in the end thereof to the surface  31   c  to be secured with the base  3   b.  The opening  32   b  is formed in the ceiling of the lens cap  32   a . The inner circumferential surface of the opening  32   b  forms a holding portion of the lens. The lens cap  32   a  is arranged so as to position the opening  32   b  thereof on the optical axis of the light emitted from the LD  25 . The optical beam focused by the Lens  32   c  is guided to the end of the optical fiber, which is not shown in the figure.  
         [0023]     The primary portion  20   a  includes a plurality of thermo-electric elements  21 , the supporting plate  22 , the photodiode (PD)  23 , the PD carrier  24 , the LD 25 , the sub-mount  26 , the thermistor  27 , and the resin  28 .  
         [0024]     The thermoelectric element  21  is, what is called, a Peltier element and is arranged between the supporting plate  22  and the sub-mount  26 . The plurality of thermoelectric elements  21  are serially connected to each other and the electrodes in each end are electrically connected to respective lead terminals  3   l a with bonding wires. The thermoelectric element  21  absorbs the heat from the supporting plate  22  where the sub-mount  26  is mounted thereon or accumulates the heat thereto. The cooling or the heating depends on the direction of the control. current. That is, the bottom plate, which supports the thermoelectric element, becomes one of the cooled or heated plates, while the other where the sub-mount  26  is mounted thereon becomes the heated or cooled plate.  
         [0025]     The sub-mount  26  is installed on the base  31   b  via the thermoelectric element  21 . The sub-mount  26  includes a primary carrier  26   a , an LD carrier  26   b , and a thermistor carrier  26   c . Among them, the primary carrier  26   a  with an L-shaped cross section has a side surface  26   d  and a mounting surface  26   e , refer to  FIG. 2 . The mounting surface  26   e  extends from the surface opposite to the side surface  26   d  in the primary carrier  26   a . The primary carrier  26   a  is made of metal such as CuW. The LD carrier  2   s b is mounted on the side surface  26   d  of the primary carrier  26   a . The LD carrier  26   b  is a slab member extending along the side surface  26   d  and is made of insulating ceramics such as AIN. The thermistor carrier  26   c  is mounted on the mounting surface  26   e  of the primary carrier  26   a . The thermistor carrier  26   a  is also a slab member and is made of insulating ceramics such as AlN.  
         [0026]     The LD  25  is fixed on the LD carrier  26   b.  Specifically, the LD  25  is arranged on a axis identical with that of the lens  31   c  such that the light-emitting surface  25   a  and the light-reflecting surface  25   b  intersect the axis X, that is, the optical axis of the light emitted from the LD  25  becomes in parallel to the axis X. The anode of the LD  25  is connected to the wiring pattern formed on the LD carrier  26   b  with a bonding wire. Similarly, the cathode of the LO  25  is connected to the other pattern formed on the LD carrier  26   b  with a bonding wire. Moreover, these wiring patterns are connected to lead terminals  31   a  with respective bonding wires. The LD  25  emits, from the light-emitting surface  25   a  thereof, the coherent light corresponding to the current supplied via the lead terminals  31   a.    
         [0027]     The thermistor  27  is a device for sensing the temperature and mounted on the thermistor carrier  26   c . On electrode of the thermistor  27  is directly connected to the wiring pattern formed on the thermistor carrier  26   c . The other electrode thereof is also connected to the other wiring pattern on the thermistor carrier  26   c . Moreover, these wiring patterns on the thermistor carrier  27  are connected to respective lead terminals  31   a  with bonding wires. The thermistor  27  changes its resistance depending on the temperature of the sub-mount  26 , which reflects the temperature of the LD  25 , and, by outputting this resistance to the outside of the module  1   a  via the lead terminal  31   a , the temperature of the LD  25  can be detected.  
         [0028]     The resin  28  as a shielding member is an article to absorb the thermal radiation from the lens cap  3 S 2   a  and the lens  32   c  to the thermistor  27 . The resin  28  is stuck to the thermistor carrier  26   a  so as to cover the thermistor  27 . The resin  28  of the present embodiment is made of resin with the good thermal conductivity. That is, the resin  28  adds thermal conductive materials such as silica (SiO 2 ) as the filler to an epoxy resin. For the filler except the silica, the aluminum nitride is well known. The thermal conductivity of the resin  29  is preferable to be greater than 1.0 W/m/K.  
         [0029]     The PD  23  is a device to detect the emission intensity of the LE)  25 , The PD  23  has light-receiving surface  23   a  that optically couples with the light-emitting surface  25   b  of the LD  25 . One electrode of the PD  23  is directly connected to the PD carrier  24 . The PD carrier  24  is connected to the lead terminal  31   a  with a bonding wire. But, the other electrode of the PD  23  is connected to another lead terminal  31   a  with a bonding wire. The PD  23  outputs a current, which corresponds to the intensity of the backlight emitted from the light-reflecting surface  25   b  of the LD  25 , to the outside of the module  1   a  via the lead terminal  31   a.    
         [0030]     The PD carrier  24  is mounted on the supporting plate  22 . The surface of the PD carrier  24  is beveled to the axis X and the PD  23  is mounted on this beveled surface. Thus, the backlight of the LD  25  reflected by the light-receiving surface  23   a  of the PD may be prevented from returning the LD  25  to cause a noise source within the LD  25 .  
         [0031]     Here, similar to the present embodiment, the PD  23  is preferable to be mounted on the supporting plate  22  not on the thermo-electric element  21 . In general, the temperature dependence of the optical sensitivity of the PD is far small compared to that of the LD under the room temperature condition between −40° C. to 85° C. Accordingly, the PD  23  is unnecessary to be mounted on the thermo-electric element, thus, the heat capacity of the members mounted on the thermo-electric element  21  may be reduced.  
         [0032]     When the LD is operated, the current supplied to the thermoelectric element is controlled in the magnitude and the direction thereof such that the resistivity of the thermistor approaches a value corresponding to the target temperature. For example, the target temperature is set for 40° C., the resistivity of the thermistor may be maintained to a value corresponding to 40° C. by constituting a feedback loop to supply the control current, which is derived from the difference between the reference corresponding to 40° C. and the signal based on the resistivity of the thermistor, to the thermoelectric element.  
         [0033]     When the ambient temperature of the module increases to, for example, 75° C., which raises the case temperature and causes the heat radiation from the case to the thermistor. Accordingly, the temperature that the thermistor practically senses becomes 40+α [° C.] by adding the contribution α [° C.] of the radiation. Therefore, to control the current in the magnitude and the direction thereof supplied to the thermoelectric element to maintain the resistivity of the thermistor to the value corresponding to 40° C. inevitably results in the excess cooling by α [° C.]. Due to this control, the emission wavelength of the LD is lengthened by A×α [nm], where A is a correlation coefficient between the temperature and the wavelength shift, from the wavelength the LD is necessary to emit. on the other hand, when the ambient temperature falls, the practical temperature that the thermistor senses becomes, by reducing the contribution P of the radiation, 40−β [° C.]. Therefore, the feedback control mentioned above results in the overheating by β [° C.], which shortens the emission wavelength of the LD by A×β [nm] from the wavelength the LD is necessary to emit.  
         [0034]     The heat radiation from the case to the thermistor strongly depends on the gap therebetween. The gap between the thermistor and the case is ensured about 3 mm in the case of the butterfly package. on the other hand, the co-axial package generally ensures only from 0.2 mm to 0.5 mm. The reason is that the butterfly package has about 10 mm square in the case size thereof, while, the co-axial package in the size thereof has a small diameter from 3 to 5 mm. Moreover, in the co-axial package the lens cap  32   a  that holds the lens  32   c  comprises a portion of the package  30 , and the LD  25  is necessary to position close to the focal point of the lens  32   c , which inevitably makes the LD  25  close to the cap  32 .  
         [0035]     The module  1   a  according to the present embodiment, the resin  28  may absorb the heat radiation from the cap  32  to the thermistor  27 . Moreover, because the resin  23  is stuck to the thermistor carrier  26   c , the resin  28  may be cooled down by the thermoelectric element  21 . Accordingly, the resistivity of the thermistor is hard to be affected from the ambient temperature of the module la, namely, from the temperature of the cap, the LD may be precisely controlled in the temperature thereof.  
         [0036]      FIG. 3A  shows a relation between the temperature of the package  30  and the shift in the emission wavelength of the LD  25 , which is called as the wavelength drift.  FIG. 3B  shows the relation between the temperature of the package and the wavelength drift without the resin  28 . These data are measured under a condition that the driving current of the LD  25  is set 40 [mA] and the current for the thermoelectric element is controlled in the magnitude and the direction thereof such that the resistivity of the thermistor  27  keeps the value corresponding to the 40 [° C.].  
         [0037]     As shown in  FIG. 3A , in the module  1   a  according to the present embodiment, the wavelength drift may be converged within 20 [pm] in the temperature range or the package  30  from −10 [° C.] to 80 [° C.] . While, in the case without resin  28  shown in  FIG. 3B , the wavelength drift of about 200 [pm] is observed in the temperature range of the package  30  from −40 [° C.] to 80 [° C.]. Thus, the module  1   a  according to the present embodiment, the temperature of the LD  25  may be precisely controlled within a vicinity of the target temperature and the wavelength drift of the emission may be reduced.  
         [0038]     Moreover, as described, the module having a co-axial package similar to that of the present embodiment is hard to arrange the structure where whole parts to be cooled down are covered because the gap between the LD and the case is quite narrow. For such situation, the module  1   a  of the present embodiment covers only the thermistor  27  by the resin  28 . Therefore, the resin may become small as compared to the aforementioned thermal sheet, which may reduce the heat capacity of the members mounted on the thermoelectric element. In the present embodiment, although the resin  28  fully covers the thermistor  27 , the resin may cover a portion of the thermistor  27  where the resin at least couples in thermal to the sub-mount  26  and positions between the thermistor  27  and the cap  32 . Moreover, in the present embodiment, the sub-mount  26  has the LD carrier  26   b  and the thermistor carrier  26   c  and the LD  26  and the thermistor  27  are independently mounted on each carrier, but the LD and the thermistor may be mounted together on the signal carrier.  
         [0039]     Moreover, in the manufacturing of the present module  1   a,  the resin  29  encapsulates the thermistor  27  and the bonding wire on the thermistor carrier  26   c  after the thermistor  27  is soldered onto the thermistor carrier  26   c  and is wire-bonded thereto after the die-bonding. Subsequently, this assembly is mounted on the mounting surface  26   e  of the primary carrier  26   a  and is wire-bonded to the lead terminals  31   a , thus, the assembly around the thermistor  27  may be simply carried out.  
       Second Embodiment  
       [0040]      FIG. 4  is a partially broken perspective view of the module  1   b  according to the second embodiment.  FIG. 5  is a cross section of the module  1   b  taken along the line V-V shown in  FIG. 4 . Differences of the module  1   b  according to the present embodiment and the module  1   a  of the first embodiment are that the module  1   b  of the present embodiment provides a slab  29  as the shielding member instead of the resin  28  of the first embodiment. The explanations of the arrangement except for the slab  29  are omitted because those are the same as the first embodiment.  
         [0041]     The primary portion  20   b  in the module  1   b  has the slab  29 . The slab  29 , positioned between the thermistor  27  and the cap  32 , shields the heat radiation from the cap  32  to the thermistor  27 . Specifically, the slab  29  is fixed in one end thereof to the upper end surface of the primary carrier  26   a , which faces the wall of the lens cap  32   a . The slab  29  extends from the upper end of the primary carrier  26   a  to protrude between the wall of the cap  32  and the thermistor  27 . The thickness of the slab  29  is, for example, 0.1 mm.  
         [0042]     The slab  29  is preferably a metal sheet made of, for example, aluminum and copper, or a metal film. Or, except for metal, the slab  29  may be made of ceramics with good thermal conductivity such as aluminum nitride. When the slab  29  is non-metallic, the surface of the slab  29  in an end portion thereon is formed with a metal pattern, and the primary carrier  26   a  made of metal and the non-metallic slab  29  may be connected via this metallic pattern.  
         [0043]     The module  1   b  of the present embodiment shields the heat radiation from the cap  32  to the thermistor  27  by the slab  29 . Moreover, the slab  29  is cooled down by the thermoelectric element  21  because the slab  29  is fixed to the primary carrier  26   a . Therefore, similar to the module  1   a  according to the first embodiment, the resistivity of the thermistor is hard to be affected by the ambient temperature of the module  1   b,  which enables to accurately monitor the temperature of the LD. Furthermore, by controlling the current supplied to the thermoelectric element  21  in the magnitude and the direction thereof based on the resistivity of the thermistor  27 , the temperature of the LD  25  may be precisely controlled to reduce the wavelength drift.  
         [0044]     The module according to the present invention is not restricted in the configuration thereof to the embodiments above described, and various modifications way be considered. For example, the resin in the first embodiment may be replaceable by various materials except for the epoxy resin. The slab in the second embodiment, which is arranged between the wall of the cap and the thermistor, may be positioned between the side of the cap and the thermistor. Or, the slab of the present invention may fully cover the thermistor. Moreover, the slab is described as independent of the primary carrier  26   a , but the slab may be a metal member integrally formed with the primary carrier  26   a  or the LD carrier  26   b.