Abstract:
The invention is directed to a laser diode assembly having a laser diode and a support, whereon the laser diode is arranged or formed in one single piece. The laser diode assembly further includes a coupling capacitor integral therewith, wherein the laser diode is subjected to the action of a laser driver circuit via the coupling capacitor associated with the assembly. The invention is also directed to a device for operating a laser diode, wherein the device includes a coupling capacitor integral with a laser diode assembly. The invention enables a reduction in the number of coupling paths between a laser driver circuit and a laser diode assembly, thereby reducing the occurrence of parasitic elements.

Description:
RELATED APPLICATION 
     This application claims priority to and incorporates by reference International Application No. PCT/DE01/03948 filed Oct. 15, 2001, which is entitled “LASER DIODE ASSEMBLY AND DEVICE FOR OPERATING A LASER DIODE”, which was not published in English, which is hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a laser diode unit with a coupling capacitance. 
     BACKGROUND OF THE INVENTION 
     In order to achieve power losses that are as small as possible, integrated circuits (ICs) for driving laser diodes are being developed for ever decreasing supply voltages. A further motivation for smaller supply voltages is constituted by breakdown criteria of many of the integrated components used. 
     The operating point of a laser diode to be driven is set by a threshold current and a modulation current, the modulation current representing the data stream. The currents generate a voltage drop across the laser diode. The voltage drop across the diode may be so large that the laser can no longer be driven directly by the laser driver IC on account of the limiting supply voltage. 
     If the voltage drop across the laser diode is so large that the laser can no longer be driven directly by the laser driver IC on account of the limited supply voltage or the required voltage drops across the driver transistors, then it is known to cause the modulation of the laser diode to be effected by means of a coupling capacitance. In the case of modulation and transmission of the data in the radiofrequency range, however, losses and mismatches due to parasitic elements occur on the coupling paths both from the modulation output of the laser driver IC to the coupling capacitor and from the coupling capacitor to the laser diode coupling unit. Said parasitic units may be, inter alia, bonding wire inductances and pad capacitances. 
     DD-A-159 584 discloses an intermittently radiating light-emitting or laser diode arrangement in which a AMENDED SHEET light-emitting diode chip is arranged on a carrier. A capacitance to which a signal is applied is situated between the carrier and the light-emitting diode chip. The carrier is an integrated circuit. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the object of providing a laser diode unit in which the occurrence of parasitic elements is reduced when using a coupling capacitance between laser driver circuit and laser diode. 
     This object is achieved according to the invention by means of a laser diode unit having at least one laser diode, an integrated circuit with a surface, on which the laser diode is arranged or formed monolithically, connection contacts for the laser diode and at least one coupling capacitance integrated into the integrated circuit. In this case, the coupling capacitance has a first metallization formed at the surface of the integrated circuit and a second metallization formed within the integrated circuit, the latter metallization being connected to a contact at a surface of the integrated circuit. A signal of a laser driver circuit is applied to the laser diode via the coupling capacitance. 
     Preferred and advantageous refinements of the invention are specified in the subclaims. 
     Accordingly, provision is thus made for the coupling capacitance arranged between the output of a laser driver circuit and the laser diode to be integrated into the laser diode unit. This has the advantage that the number of problematic coupling paths from the laser driver circuit through to the laser diode unit is reduced and the occurrence of parasitic elements is reduced. Thus, a separate coupling path between the laser diode unit and an external coupling capacitance, as is provided in the prior art, is obviated on account of the invention&#39;s integration of the coupling capacitance into the laser diode unit. This advantage may be utilized both for single-ended and for differential driving of a laser diode or a laser diode array. 
     It is thus possible for the modulation outputs of a laser driver circuit which is generally formed as an integrated circuit, to be directly connected to the laser diode unit, for example by bonding wires or by means of chip-on-chip mounting. 
     A laser diode array is understood to be an arrangement of a plurality of laser diodes in a mechanical interconnection. The laser diode unit is formed for this case by the laser diode array, a carrier for the laser diode array, electrical connection contacts and at least one coupling capacitance. 
     A preferred refinement of the invention provides for the laser diode to be placed by one connection directly (e.g. by adhesive bonding) onto a metallization on the surface of the integrated circuit, which forms a contact of the coupling capacitance. The other connection of the laser diode is preferably realized by a short bonding wire. 
     In an advantageous development of the invention, the laser driver circuit is also integrated into the integrated circuit or the integrated circuit is the laser driver IC, i.e. the at least one coupling capacitance is concomitantly integrated into the laser driver IC. The laser diode or the laser diode chip is mounted directly on the laser driver IC, the laser diode being connected to the metallization of the laser driver IC by at least one connection and the other connection being realized by a particularly short bonding wire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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  schematically shows the construction of a first exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the laser diode unit having a p-contact laser diode chip with single-ended driving; 
         FIG. 2  shows an exemplary embodiment corresponding to  FIG. 1  with an n-contact laser diode chip; 
         FIG. 3  schematically shows the construction of a second exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the laser diode unit having a p-contact laser diode chip with single-ended driving; 
         FIG. 4  shows an exemplary embodiment corresponding to  FIG. 3  with an n-contact laser diode chip; 
         FIG. 5  schematically shows the construction of a third exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the laser diode unit having a p-contact laser diode chip with single-ended driving; 
         FIG. 6  shows an exemplary embodiment corresponding to  FIG. 5  with an n-contact laser diode chip; 
         FIG. 7  schematically shows the construction of a fourth exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the laser diode unit having a p-contact laser diode chip with differential driving; 
         FIG. 8  shows an exemplary embodiment corresponding to  FIG. 7  with an n-contact laser diode chip; 
         FIG. 9  schematically shows the construction of a fifth exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the laser diode unit having a p-contact laser diode chip with differential driving; 
         FIG. 10  shows an exemplary embodiment corresponding to  FIG. 9  with an n-contact laser diode chip; 
         FIG. 11  schematically shows the construction of a sixth exemplary embodiment of a laser diode unit with an integrated coupling capacitance, the coupling capacitance being integrated into a laser driver IC and a p-contact laser diode chip with differential driving being used, and 
         FIG. 12  shows an exemplary embodiment in accordance with  FIG. 11  with an n-contact laser diode chip. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with  FIG. 1 , a laser diode chip  1 , also referred to as laser diode, a coupling capacitance  2 , a carrier/submount  3  and also associated electrical connection contacts form a laser diode unit  100 . The laser diode unit may optionally additionally be surrounded by a housing (not illustrated) and be encapsulated with respect to the surroundings by said housing. 
     In the exemplary embodiment of  FIG. 1 , the coupling capacitance  2  is arranged between the submount  3  and the laser diode  1 , i.e. the coupling capacitance  2  is mounted directly on the submount  3  and the laser diode  1  is mounted directly onto the coupling capacitance  2 . 
     The laser diode  1  has in a manner known per se, a semiconductor laser structure  13 , an upper n-contact  11  and a lower p-contact  12 . The two contacts  11 ,  12  are formed by metallic areas. The laser diode  1  is preferable a vertically emitting diode. In principle, however, it is possible to use any desired laser diodes. 
     The coupling capacitance  2  is formed as a chip capacitance. The capacitor areas of the coupling capacitance  3  are formed by metallized contact areas  21 ,  22  at the opposite sides of a chip  23  placed onto the submount  3 . The submount  3  is a high-impedance substrate with a low parasitic capacitance. 
     As can be gathered from the circuit diagram on the right-hand side of  FIG. 1 , the laser diode  1  experiences single-ended driving with a radio frequency (RF) modulating current which is modulated in accordance with a data signal to be transmitted. The RF modulation current is generated by a laser diode driver circuit and applied to the lower contact  22  of the coupling capacitance  2  via a bonding wire, by way of example. A bias current is applied to the laser diode  1  via the upper contact  21  of the coupling capacitance  2 . In this case, the bias current is large enough that the laser diode  1  can be operated directly even in the case of a small supply voltage of the laser diode driver circuit of 3.3 V, by way of example. 
     The upper contact  21  of the coupling capacitance  2  is simultaneously the p contact  12  of the laser diode chip  1 . The n-contact  11  of the laser diode chip  1  is fitted on the upper side of the laser diode chip  1  and connected to an electrical reference potential. 
     Contact is made with the laser diode unit  100  by means of a direct connection of the corresponding outputs of the laser driver circuit to the contacts  22 ,  21  of the coupling capacitance  2  and, respectively, the contacts  11 ,  12  of the laser diode chip  1 , for example by means of bonding. It is thus possible to connect the outputs of the laser driver circuit directly to the laser diode unit. 
     The exemplary embodiment of  FIG. 2  involves, instead of a p-contact laser diode, an n-contact laser diode, the lower contact  12  of which is the p-contact and the upper contact  11  of which is the n-contact. Otherwise, there are no differences with respect to the embodiment of  FIG. 1 . 
     In the exemplary embodiment of the  FIG. 3 , the coupling capacitance  2   a  is again formed as a chip capacitance with an upper contact  21   a  and a lower contact  22   a . However, the coupling capacitance  2   a  is arranged next to the p-contact laser diode chip  1   a  on the submount  3   a . In this case there is provided on the surface of the submount  3   a  a continuous contact-connection  31   a , which comprises both the lower contact-connection  22   a  of the coupling capacitor  2   a  and the n-contact  12   a  of the laser diode  1   a  and via which the bias current is applied. The RF modulation current is fed via the upper contact  21   a  of the coupling capacitance. The p-contact  11   a  of the laser diode  1   a  is again connected to an electrical reference potential. 
     For the rest, reference is made to the explanations concerning  FIG. 1 . In the exemplary embodiment of  FIG. 4 , the only difference with respect to the exemplary embodiment of  FIG. 3  again consists in the formation of the laser diode as an n-contact laser diode. The common contact-connection  31   a  is correspondingly located at the p-contact of the laser diode chip  1   a.    
     The exemplary embodiment of  FIG. 5  shows an alternative exemplary embodiment to the exemplary embodiment of  FIG. 3  insofar as no common contact-connection is provided on the surface of the submount  3   a . In this exemplary embodiment, the radiofrequency signal is applied to a contact pin  31   a   1  on the submount  3   a , which is connected to the lower contact  22   a  of the coupling capacitance  2   a . The bias current is applied to the upper contact  21   a  of the coupling capacitance  2   a . Contact is made with the p-contact  11   a  of the laser diode chip  1   a  via a short bonding wire  4  from the upper contact  21   a  of the coupling capacitance  2   a . The lower n-contact  12   a  of the laser diode  1   a  is connected to an electrical reference potential via a further contact pin  31   a   2 . Consequently, the p-contact  11   a  is the “hot” contact in this exemplary embodiment. 
     The connection of the contacts of the laser diode unit to the corresponding outputs of the laser driver circuit is preferably again effected via bonding wires. 
       FIG. 6  shows the construction of  FIG. 5  in the case of a laser unit with an n-contact laser diode chip  11   a.    
       FIGS. 1 to 6  have illustrated circuit arrangements in which a laser diode has been driven in single-ended fashion. The subsequent exemplary embodiments of  FIGS. 7 to 12  show arrangements with differential driving of a laser diode. In this case, a laser driver circuit in each case feeds a differential modulation current via a contact RF-A and a contact RF-B and the bias-current current supply is realized via contacts bias-A and bias-B have mutually inverse signals, so that the laser diode is driven with a doubled amplitude. 
     In the exemplary embodiment of  FIG. 7 , two coupling capacitors  2   b   1 ,  2   b   2  are arranged next to one another on a submount  3   b . A p-contact laser diode chip  1   b  is arranged above one coupling capacitor  2   b   2  in a manner similar to that in  FIG. 1 . A short bonding wire  4  extends from the top p-contact  11   b  of the laser diode chip  1   b  to the upper contact or to the upper capacitor area  21   b   1  of the other coupling capacitor  2   b   1 . The RF modulation current RF-A is present at the lower contact  22   b   2  of the left-hand coupling capacitor  2   b   1  and the RF modulation current RF-B is present at the lower contact  22   b   1  of the right-hand coupling capacitor  2   b   2 . The bias currents bias-A and bias-B are fed via the respective other contacts  21   b   1 ,  21   b   2  of the coupling capacitors. 
       FIG. 8  shows the same situation in the case of an n-contact laser diode chip. 
     In the exemplary embodiment of  FIG. 9 , two coupling capacitors  2   c   1 ,  2   c   2  and a laser diode chip  1   c  are in each case arranged directly on a submount  3   c , the laser diode chip  1   c  being arranged between the two coupling capacitances  2   c   1 ,  2   c   2 . The submount  3   c  has two mutually separate contact areas  31   c   1 ,  31   c   2 , one coupling capacitor  2   c   1  being arranged on one contact area  31   c   2  and the other coupling capacitor  2   c   2  and the laser diode  1   c  being arranged on the other contact area  31   c   2  by their respective contacts  22   ca ,  22   c   2 ,  12   c . A connection between the laser diode  1   c  and the left-hand coupling capacitance  2   c   1  is again effected via a short bonding wire  4 . 
     The RF modulation currents RF-A, RF-B are fed via the lower contact  22   c   1  of the left-hand coupling capacitance  2   c   1  and the upper contact  21   c   2  of the right-hand coupling capacitance  2   c   2 , and the bias currents bias 1 , bias  2  are fed via the upper contact  21   c   1  of the left-hand coupling capacitance  2   c   1  and one contact area  31   c   2  of the submount  3   c , which is connected to the lower n-contact  12   c  of the laser diode  1   c.    
       FIG. 10  again shows the corresponding arrangement when using an n-contact laser diode chip. 
     It is pointed out that, in the exemplary embodiments of  FIGS. 1 to 10 , the coupling capacitance or the coupling capacitances may also be integrated monolithically into the submount instead of as a chip capacitance. In this case, the laser chip  1  is mounted on the submount. This embodiment variant has the advantage that separate production and mounting of the coupling capacitances is not necessary and a higher integration density is achieved. In principle, it is likewise conceivable for the laser diode also to be formed monolithically with the other components. 
       FIG. 11  shows a further exemplary embodiment, in which two coupling capacitors  2   d   1 ,  2   d   2  are integrated into a carrier  3   d , which constitutes a chip with an integrated circuit and contains, in particular, a laser driver circuit  5 , to which two mutually inverted input signals D, DN are fed and which generates therefrom modulation currents RF-A, RF-B and bias currents bias-A, bias-B for the differential driving of the laser diode  1   d.    
     The coupling capacitors  2   d   1 ,  2   d   2  in each case have a metallization  21   d   1 ,  21   d   2  formed at the surface of the integrated circuit or laser driver IC and a metallization  22   d   1 ,  22   d   2  formed within the laser driver IC, the latter metallization in each case being led to the chip surface via a vertical connection and forming contacts  22   d   1 ′,  22   d   2 ′ there. 
     The laser diode chip  1   d  is mounted directly (chip-on-chip) onto the integrated circuit or laser driver IC  3   d . In this case, the lower n-contact  12   d  of the laser diode  1   d  is directly connected to a metallization on the laser driver IC which forms one metallization and contact area  21   d   2  of the right-hand coupling capacitor  2   d   2 . By way of example, the laser diode  1   d  is connected to the right-hand coupling capacitance  2   d   2  by adhesive bonding. 
     The other connection of the laser diode  1   d  is realized by means of a short bonding wire to one contact  21   d   1  of the left-hand coupling capacitance  2   d   1 . 
     The RF modulation currents RF-A, RF-B are fed via the contact  22   d   1 ′ of the left-hand coupling capacitance  2   d   1 , said contact being connected to the lower metallization  22   d   1 , and the contact  22   d   2 ′ of the right-hand coupling capacitance  2   d   2 , said contact being connected to the lower metallization  22   d   2 , and the bias currents bias 1 , bias  2  are fed via the upper contact  21   d   1  of the left-hand coupling capacitance  2   d   1  and the upper contact  21   d   2  of the right-hand coupling capacitance  2   d   2 . 
     By virtue of the coupling capacitances being integrated into the laser driver IC  5 , it is possible to effect very short bonding connections (not illustrated) between the laser driver IC  5  and the laser diode  1   d  or the coupling capacitors  2   d   1 ,  2   d   2 , thereby further reducing the occurrence of undesirable parasitic capacitances or inductances. 
     Finally,  FIG. 12  shows the arrangement of  FIG. 11  for the case where an n-contact laser diode is used, the same construction being used. 
     The embodiment of the invention is not restricted to the exemplary embodiments illustrated above. 
     For example, the specific arrangements and interconnections of the submount, coupling capacitances and laser diode are to be understood only by way of example. It may also be provided that coupling capacitances are integrated into a laser diode unit in which a plurality of laser diodes are arranged in a laser diode array in a manner known per se. 
     All that is essential to the invention is that the coupling capacitance or the coupling capacitances are integrated into the laser diode unit.