Patent Publication Number: US-2005138934-A1

Title: Optoelectronic component with a peltier cooler

Description:
It is known to use Peltier coolers for stabilizing the temperature of optoelectronic devices. Peltier coolers exploit the Peltier effect, according to which heat is drawn from or fed to the interface between two different conductors when current flows, depending on the current direction. Usually, two semiconductor materials having a different conduction type are connected to one another with a readily conductive metal bridge that forms the cooled area.  
      The known Peltier coolers used for stabilizing the temperature of optoelectronic devices are usually incorporated in a comparatively large housing, for example a so-called butterfly housing, on account of their size.  
      The present invention is based on the object of providing a compact optoelectronic assembly which can be used even in housings of small design. In addition, the intention is to enable an as far as possible temperature-insensitive coupling of an optical waveguide to the assembly.  
      This object is achieved according to the invention by means of an optoelectronic assembly having the features of claim  1 . Preferred advantageous refinements of the invention are provided in the subclaims.  
      Accordingly, the solution according to the invention is distinguished by the fact that the cooling element used is a Peltier cooler having a thickness of less than 1 mm, on which the component is arranged either directly or with interposition of a carrier substrate for the optoelectronic or passive optical component.  
      This results in a very compact construction that makes it possible to integrate the Peltier cooler together with the optoelectronic or passive optical component and also further components, if appropriate, into a hermetically sealed housing of small design. In this case, it is possible to realize very small optoelectronic constructional forms, for example TO constructional forms. In particular, it is possible to realize temperature-stabilized ITU laser sources having a constant wavelength at different ambient temperatures in small constructional forms.  
      The Peltier cooler is preferably embodied in silicon, silicon carbide, diamond or another material having high thermal conductivity. In this case, the Peltier cooler advantageously has the same or virtually the same coefficient of thermal expansion as the optoelectronic or passive optical component arranged thereon or the carrier substrate, which are usually formed in silicon. Consequently, only very low thermal stresses arise. As a result, it is possible to effect a stable, temperature-insensitive single-mode coupling with an optical waveguide to be coupled.  
      Moreover, Peltier coolers comprising corresponding materials, in particular silicon-based Peltier coolers, expand to a lesser extent than conventional Peltier coolers, thereby making it possible to keep the position of a radiation source, for instance of a laser, stable with regard to a fiber to be coupled. It is thus possible, e.g. to effect an adjustment at room temperature while the radiation source is being operated in operation at a different temperature.  
      In the case where the optoelectronic or passive optical component is arranged directly on the Peltier cooler, the latter additionally performs the functions of a carrier or submount, so that a separate carrier can advantageously be dispensed with. Very compact arrangements thus result.  
      The Peltier coolers used are, in particular, so-called micro-Peltier coolers, having a high cooling capacity in conjunction with a small area and short response times. Production takes place by means of methods appertaining to thin film technology and Microsystems technology. For cost-effective fabrication, the micro-Peltier coolers are processed on standard silicon wafers and then separated. The micro-Peltier coolers have a thickness of less than one millimeter. The edge length is preferably less than 5 mm, and in particular is 1-2 mm. The thermoelectric functional materials are structured vertically and originate for example from the family of bismuth chalcogenides.  
      In a preferred refinement, the solution according to the invention is distinguished by the combination of a micro-Peltier cooler with a small constructional form for an optoelectronic assembly, for instance a small TO constructional form or a comparable small, hermetically sealed constructional form. A compact construction of Peltier cooler and an optoelectronic or passive optical component that is to be stabilized in terms of temperature is provided.  
      In a preferred refinement, the arrangement is constructed in such a way that the optical axis of the optoelectronic transmitting and/or receiving element is perpendicular to the Peltier cooler. A particularly compact construction is provided as a result of this.  
      In an advantageous embodiment, the Peltier cooler is provided with solderable metalization that can be patterned highly precisely by means of photolithographic methods. Via the metalization, it is possible to make contact with optoelectronic components arranged on the Peltier cooler directly. In this case, one contact of the optoelectronic component is soldered for example to metalization on the Peltier cooler, while the other contact is contact-connected by means of a bonding wire.  
      The Peltier cooler furthermore preferably has micromechanical trenches, which serve in particular for receiving an optical fiber. The trenches are preferably V-grooves etched into silicon in the 110 plane. Further structures for instance for self-alignment processes may likewise be formed on the Peltier cooler.  
      Moreover, additional components may be arranged on the Peltier cooler, for instance an additional monitor diode for monitoring the laser light and/or a temperature diode for monitoring the temperature, and also glass prisms for beam deflection and lenses. In this case, the Peltier cooler provides for temperature stabilization of the entire arrangement.  
      It is further pointed out that constructions of one- or two-dimensional arrays of diodes, for instance VCSEL diodes, may also be arranged on the Peltier cooler directly or with interposition of a carrier substrate.  
      The optoelectronic component is preferably a transmitting and/or receiving unit for optical message transmission. An optical component arranged on the Peltier cooler is for example a WDM filter, a multiplexer/demultiplexer or a switch.  
      In a further embodiment variant, optical and/or electrical components, for example a diode or a thin-film resistor, are integrated directly into the Peltier cooler. The degree of integration of the assembly is increased further as a result of this.  
      In one refinement of the invention, a specific Peltier cooler that provides a specific temperature regulation is in each case provided for individual components or component arrangements of the assembly. The individual specific Peltier coolers may in turn be connected to a large, conventional Peltier cooler, the specific Peltier coolers then being responsible for fine regulation.  
      The invention is preferably used in conjunction with passive optical components which inherently have no evolution of heat. Instead of temperature stabilization, the Peltier coolers may in this case also serve for influencing optical signals in a defined manner. Thus, a local temperature change that leads to a phase change may be brought about by means of Peltier coolers in particular in the case of optical modulators such as, for example, Mach-Zehnder interferometers or directional couplers. In particular, micro-Peltier elements can replace strip heaters used in the prior art in passive optical components appertaining to optoelectronics. In this case, a micro-Peltier element is assigned for example to an optical waveguide or optical waveguide arm of an optical modulator, the phase of the light in the optical waveguide or optical waveguide arm being set in a defined manner by means of heating or cooling.  
      In an advantageous further refinement, a plurality of Peltier elements are arranged in a Peltier array. In this case, the Peltier array is assigned for example to an array of passive optical elements, for instance an array of Mach-Zehnder inteferometers, and in each case provides locally for a desired temperature change. 
    
    
      The invention is explained in more detail below using a plurality of exemplary embodiments with reference to the figures of the drawing, in which:  
       FIG. 1  shows a diagrammatic illustration of a TO housing with a silicon chip arranged on a micro-Peltier cooler;  
       FIG. 2  shows one beside the other, a transmitting and receiving element in each case arranged in a TO housing in an arrangement in accordance with  FIG. 1 ;  
       FIG. 3  shows a plan view of an exemplary embodiment of a transmitting assembly mounted on a Peltier cooler;  
       FIG. 4  shows an exemplary embodiment of a VCSEL laser mounted on a micro-Peltier cooler;  
       FIG. 5  shows an exemplary embodiment of an edge emitting laser with integrated beam deflection mounted on a micro-Peltier element;  
       FIG. 6  shows a detail view of the beam deflection of the arrangement of  FIG. 5 ;  
       FIG. 7  shows an arrangement in which an edge emitter laser, rotated through 90°, is coupled to a heat sink by means of a micro-Peltier element;  
       FIGS. 8   a - b  diagrammatically show the construction of a temperature-stabilized transmitting assembly in accordance with the prior art in side and plan views;  
       FIGS. 9   a - b  diagrammatically show the construction of a temperature-stabilized transmitting assembly according to the invention in side and plan views;  
       FIG. 10  shows a micromodule with double beam deflection arranged on a micro-Peltier element;  
       FIGS. 11   a - c  show a temperature-stabilized transmitting assembly with an edge emitter in side and plan views and with a sectional view of an integrated v-groove formed in an SI chip;  
       FIGS. 12   a - b  show an arrangement corresponding to  FIGS. 11   a - c  with a V-groove integrated into the Peltier element;  
       FIGS. 13   a - d  show the arrangement of an edge emitter arranged on a Peltier element with two configurations of the edge emitter;  
       FIGS. 14   a - b  show the arrangement of a VCSEL laser diode arranged on a Peltier element in side and plan views;  
       FIG. 15  shows a fiber Bragg filter arranged on a Peltier element;  
       FIG. 16  diagrammatically shows the arrangement of a passive optical component on a Peltier element; and  
       FIG. 17  shows the arrangement of a micro-Peltier cooler in a Mach-Zehnder interferometer. 
    
    
       FIG. 1  diagrammatically shows an exemplary embodiment of an optoelectronic transmitting and/or receiving element  1  arranged on a micro-Peltier cooler  2 .  
      The transmitting and/or receiving element is formed as a chip  1  having for example a laser, in particular a VCSEL laser, a photodiode or a silicon micromodule with transmitting and monitor diode and optical deflection means. The chip  1  is arranged directly on the micro-Peltier cooler  2 , which in this case simultaneously serves as a carrier substrate (submount). Both are situated in a TO housing  3 , to be precise a TO housing of small design, which has a cap  31 . In this case, the optical axis of the chip  1  runs perpendicular to the micro-Peltier cooler  2 .  
      TO (Transistor Outline) housings are standard housings known in the prior art for optical transmitting or receiving modules, the form of which is similar to the housing of a (traditional) transistor but which have a glass window for entry and exit of light at the top side. There are standardized sizes for TO housings. Small TO housings of the TO46, TO35, TO37 and TO52 standard, for example, are used in the present case, the numerical indication specifying the external diameter.  
      The micro-Peltier element  2  is embodied in silicon and likewise has small dimensions. It has a thickness of less than 1 mm and an edge length of 1-2 mm, for example. As an alternative, the micro-Peltier element  2  may also comprise silicon carbide, diamond or other materials having high thermal conductivity.  
      At its top side, the cap  31  has a TO window and a fiber coupling  32  and/or a filter element. The micro-Peltier cooler  2  is mounted on a base plate  33  through which pass terminal pins  34  of the TO housing  3 . The chip  1  is contact-connected by means of bonding wires  4 , one bonding wire being led from one contact pin directly to a terminal pad on the top side of the chip  1 , while the other bonding wire is connected to a terminal pad on the top side of the micro-Peltier cooler  2 . In this case, the micro-Peltier cooler  2  has solderable metalization in particular gold metalization, which can be patterned highly accurately by means of photolithography. The underside of the chip  1  is contact-connected via the solderable metalization.  
       FIG. 2  shows an arrangement in which two TO assemblies  5 ,  6  in accordance with  FIG. 1  are arranged one beside the other in a transceiver. In one TO assembly  5 , a transmitting element  51  is arranged directly on a micro-Peltier cooler  52 ; in the other TO assembly  6 , a receiving element  61  is arranged directly on a micro-Peltier cooler  52 . The distance A is only approximately 5-10 mm on account of the small dimensions of the TO housings. The arrangement may thus be used as a subassembly in an optoelectronic transceiver of small design.  
       FIG. 3  shows an example of the concrete construction of the transmitting assembly of a TO housing in accordance with  FIG. 1 . Accordingly, a chip  1  is arranged directly on the micro-Peltier element  2 , said chip having a micromodule with a laser diode, a monitor diode  11  and a temperature diode  12 . The laser diode is concealed by a lens  7  in the plan view illustrated. The illustration likewise shows the respective bonding wires  81 - 86  for making contact with the individual components.  
      The monitor diode  11  serves in a customary manner for detecting and monitoring the power radiated by the laser diode. On account of its proximity to the transmitting diode, the temperature diode  12  specifies the temperature of the transmitting diode. In this case, the signal generated by the temperature diode  12  serves for regulating the Peltier element  2 , i.e. this is cooled or heated depending on the temperature stabilization to be effected.  
      As an alternative, the use of a separate temperature diode may also be dispensed with and the monitor diode may be used for temperature measurement. It is also pointed out that the components illustrated do not have to be integrated into a micromodule  1  that is then arranged on the micro-Peltier element  2 . Instead, laser diode, monitor diode and temperature diode may in each case also be arranged directly on the micro-Peltier element  2 . In this case, the monitor diode  11  and the temperature diode  12  may be positioned discretely on the silicon Peltier cooler  2  or, in the event of being a silicon diode, may be integrated directly into the silicon Peltier cooler  2 .  
      In particular, a diode and/or further components such as a thin film resistor may be integrated in the upper or lower cover of the silicon Peltier cooler  2 .  
       FIG. 4  shows a coaxial construction of a transmitting assembly, a VCSEL laser chip  9  being arranged directly on a micro-Peltier cooler  2 . In this case, the upper contact of the laser chip  9  is provided by a bonding wire  4  proceeding from the surface of the micro-Peltier cooler  2 . Further bonding wires connect the contact pins  34  of the TO housing (illustrated incompletely) to contact pads or metalizations on the surface of the micro-Peltier cooler  2 . The micro-Peltier element  2  in turn serves as a submount for the laser chip  9 .  
       FIGS. 5 and 6  show an arrangement that is comparable to the arrangement of  FIG. 4 , an edge emitting laser being used instead of a VCSEL laser chip. In this case, a beam deflection is integrated in the laser chip  10 , said beam deflection being provided by a crystallographically etched mirror area  11  and deflecting the laterally radiated laser beam perpendicularly upward.  
      The exemplary embodiment of  FIG. 7  illustrates an edge emitting laser chip  13  arranged in a TO housing (again illustrated only partially) in an arrangement rotated through 90° relative to  FIG. 5 . In this case, the laser chip  13  is positioned directly on a micro-Peltier element  2 , which is in turn mounted on a heat sink  12  integrated in the TO housing.  
       FIGS. 8   a  and  8   b  show the known construction of a construction—used for optical data transmission—with an edge emitting laser chip  14 , a monitor diode  15 , a temperature diode  16  (which is embodied for example as a thermistor), a carrier substrate  17 , made in particular, of silicon, on which the above-mentioned elements  14 ,  15 ,  16  are arranged, a lens  18 , a filter  19  or optical isolator and an optical waveguide  20 , in which light from the laser  14  is coupled. The arrangement is arranged altogether on a common Peltier element  21 , which is in turn coupled to a heat sink  22 . It is disadvantageous that specific thermal regulation of the individual elements cannot be effected in this case.  
       FIGS. 9   a ,  9   b  show an arrangement in which the laser chip  14 , the monitor diode  15 , the temperature diode  16  and the corresponding carrier substrate  17  are arranged on a specific micro-Peltier cooler  23 . Specific temperature regulation can now be effected. If appropriate, a conventional, large Peltier element may additionally be used for the entire arrangement, in which case the micro-Peltier cooler  23  would then be responsible for fine regulation.  
      As an alternative, the carrier substrate  17  may also be dispensed with and the elements  14 ,  15 ,  16  may be arranged directly on the micro-Peltier cooler  23 .  
       FIG. 10  shows an exemplary embodiment of a micromodule  24 , which is again arranged on a micro-Peltier cooler  23  formed in silicon. The micromodule has a laser  25 , a monitor diode  26 , a silicon lens  27  and two glass prisms  28 ,  29  for double beam deflection. For passive mounting of the components, alignment marks may be integrated into the micro-Peltier cooler. The micro-Peltier cooler may also have micromechanical cut-outs for forming receptacle structures for the components.  
      The exemplary embodiment of  FIG. 10  represents an example of the arrangement of various optical and optoelectronic components on a micro-Peltier cooler. As an alternative, the arrangement is connected to the micro-Peltier cooler by means of an additional submount.  
      The exemplary embodiment of  FIGS. 11   a - 11   c  is similar to the exemplary embodiment of  FIGS. 9   a ,  9   b , the optical fiber  20  being arranged in a V-groove  31  of silicon chip  30  adjoining the micro-Peltier cooler  23 . In this case, the fiber  20  goes directly right into the laser  14  by means of a butt coupling. Instead of being arranged in an optical fiber  20 , the light may also be arranged in an integrated waveguide embodied for example using glass on silicon technology. In this case, the integrated waveguide formed on the chip  30  is likewise brought directly right up to the laser  14 .  
      The arrangement illustrated permits a specific cooling only of the component group  14 ,  15 ,  16 . It is not necessary to arrange the entire assembly on a Peltier element as in the prior art (cf.  FIG. 8 ). Since the micro-Peltier cooler  17  is preferably formed from silicon, it has similar thermal properties to the silicon chip  30 . As a result, optical waveguide  20  and laser chip  14  can be aligned with respect to one another without the silicon chip  30  also being temperature-stabilized.  
      In  FIGS. 12   a ,  12   b , the submount  17  arranged on the micro-Peltier cooler  23  additionally performs the function of the silicon chip  30  of  FIG. 11 , a V-groove being micromechanically integrated into the submount. If no submount is provided, the V-groove is introduced directly into the micro-Peltier cooler.  
       FIGS. 14   a - 14   d  show a construction with an edge emitting laser  14 , into which, in accordance with  FIG. 13   c , an etching trench  31  with a mirror area  32  is integrated, through which the light is radiated upward. In the example of  FIG. 13   d , this is achieved by means of a mirror area  33  given upside—down mounting of the laser diode  14 .  
      Generally, provision may be made of external prisms/mirrors or integrated arrangements for beam deflection. The latter may, however, also be monolithically integrated into the micro-Peltier element.  
      In accordance with  FIGS. 14   a ,  14   b , a vertically emitting laser  14  with an active laser region  14   a  is mounted on a micro-Peltier cooler  23  by means of a submount  17  or directly. A monitor diode  16  serves for temperature regulation. Such a construction is particularly compact.  
      In a manner analogous to that described with reference to the above figures, receiving elements may also be coupled to a micro-Peltier cooler. This may involve receiver diodes whose light-sensitive area is situated on the top side or alternatively on the underside, or else laterally illuminated receiver diodes, in particular those for high data rates above 10 Gbit/s.  
      By way of example, use with a silicon avalanche photo diode (APD) is advantageous. In the case of construction on a Peltier cooler, the signal-to-noise ratio can be improved by means of a temperature regulation. In the case of APD diodes, the avalanche factor is temperature-dependent. In the case of an APD array, the individual pixels could be regulated to different temperatures by means of a Peltier array in order thus to compensate for the fluctuations in the gain factor, or to set different gain factors in a specific manner.  
      The use of a micro-Peltier cooler  23  is also of interest in conjunction with passive optical components, in particular of a WDM (wavelength division multiplex) system, since they are considerably more compact than conventional arrangements and actually enable specific temperature regulation of individual components. Such components, for instance filters, multiplexers, must likewise be temperature-stabilized.  
      In accordance with  FIG. 15 , a Fabry-Perot filter  34  formed in a waveguide is coupled to a micro-Peltier cooler  23  by means of a submount  17 .  FIG. 16  generally shows a diagrammatically illustrated passive optical component  35  on a micro-Peltier cooler  23 , it being possible for a submount  17  additionally to be provided. However, in this case, too, the micro-Peltier cooler may simultaneously serve as a submount.  
       FIG. 17  illustrates a Mach-Zehnder interferometer  36  such as is employed in WDM systems. The signals of a plurality of data channels which are transmitted in an optical waveguide  38  are present at the input  37  of the Mach-Zehnder interferometer  36 . In this case, the individual data channels each have a different wavelength. By way of example, the wavelengths of the data channels lie in the range between 1530 nm and 1570 nm. In the frequency domain, the channel spacing is 100 GHz, for example.  
      The Mach-Zehnder interferometer  36  operates as a spectral filter. A coupler is present at its input  37  and divides the input signal between two arms  36   a ,  36   b  of the filter  36 . In order to be able to precisely set the phase difference between the two arms  36   a ,  36   b , a phase shifter  39  is connected to the lower arm  36   b . Instead of the heating electrodes or strip heaters known in the prior art, a micro-Peltier element  39  is used as the phase shifter. The waveguide  36   b  can be locally cooled or heated by means of a cooling or heating. By means of the thermo-optical effect, this process of cooling or heating causes a change in refractive index, so that the optical path length can be set by means of the micro-Peltier element  39  and a phase shift can thus be generated between the signals of the two arms  36   a ,  36   b . As a result, the filter properties of the filter  36  can be configured as desired within a wide range and be designed for a wide variety of applications. In particular, the filter is designed in such a way that, at the output  40  of the Mach-Zehnder interferometer  36 , the signals are distributed between two output arms in a wavelength-dependent manner.  
      It is equally conceivable for the Mach-Zehnder interferometer  36  to represent part of an attenuator unit. The incoming signals are divided between the two arms  36   a ,  36   b  and combined again after a phase shift in one arm, as a result of which a defined signal attenuation can be set.  
      The exemplary embodiment of  FIG. 17  is only a representative example of configurations in which a phase change in a signal is brought about by means of a micro-Peltier cooler. Other examples are directional couplers, optical switches and optical multiplexers/demultiplexers. In most cases, conventionally used heating electrodes may likewise be replaced by a micro-Peltier element in each case. On account of its small size, a micro-Peltier element ensures in this case that a temperature change occurs only in a locally delimited region.