Abstract:
An electro-optical assembly, consisting of an optical sub-assembly and a transmission line. The optical sub-assembly consists of an electro-optical component having an optical region and a first and a second electrode coupled thereto, and a conductive optical bench in contact with the second electrode of the electro-optical component, the optical bench being adapted to permit optical alignment of the electro-optical component while making such contact. The transmission line consists of a live conductor, a ground conductor insulated from the live conductor, and a port adapted to receive a signal. The live and ground conductors are coupled to the first and second electrodes of the electro-optical component so as to convey the signal between the port and the electro-optical component and to provide a direct current (DC) bias level to the electro-optical component independent of the signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/314,321, filed Aug. 23, 2001, which is assigned to the assignee of the present invention and which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to electro-optical components, and specifically to electrical biasing of the components.  
         BACKGROUND OF THE INVENTION  
         [0003]    In order for an electro-optical component, such as an Electro-Absorption Modulator (EAM) or a photo-diode detector (PDD), to function correctly, the element typically requires the ability to be independently DC biased and AC modulated. In addition to these electrical requirements, the element usually also needs to be aligned optically.  
           [0004]    [0004]FIG. 1 illustrates apparatus, known in the art, for electrically and optically coupling an electro-optical component  10 . Component  10  is mounted on a first capacitor  24 , which typically has a capacitance of the order of 1 nF, so that a lower electrode  17  of the component mates with a first electrode  19  of the capacitor. Capacitor  24  is in turn mounted on a conductive optical bench  26 , so that a second electrode  11  of the capacitor is in electrical contact with the optical bench. A second capacitor  28 , typically having a capacitance of the order of 1 μF, is coupled in parallel with capacitor  24 , the two capacitors forming a low impedance path for low and high AC frequencies between the optical bench and the optical element. Optical bench  26  is electrically connected to a ground conductor  12  of a transmission strip-line  16 . A second conductor  14  of the transmission strip-line is bonded, by a wire  18 , to an upper electrode  22  of component  10 . Typically, a resistor  21  may be connected between electrode  22  and ground  12 . The resistor serves as an impedance match and as a DC return.  
           [0005]    Electro-optical component  10  is aligned with an optical element  20 , such as a fiber optic, by adjusting optical element  20 . When alignment is achieved, optical element  20  is mechanically coupled to the optical bench.  
           [0006]    The arrangement of elements as shown in FIG. 1 provides DC isolation of electro-optical component  10  from ground conductor  12 , while enabling the component to be modulated by an AC voltage via capacitors  24  and  28 . Thus, electro-optical component  10  may be DC biased independent of any AC modulation provided to the element, by applying a DC bias level to electrode  17  and applying a ground potential to electrode  22  via resistor  21 . However, this method of arranging elements in order to be able to DC bias electro-optical component  10  separates the component from optical bench  26 , causing severe difficulties in aligning the component. An improved arrangement for aligning an electro-optical component is thus required.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of some aspects of the present invention to provide a method and apparatus for biasing and optically aligning an electro-optical component.  
           [0008]    In preferred embodiments of the present invention, an electro-optical assembly comprises a transmission line which is coupled to an electro-optical component. The electro-optical component comprises a first and a second electrode coupled to an optical region, the optical region being aligned optically. The transmission line, preferably a micro-strip line, comprises a “live” conductor and a ground conductor, the ground conductor being divided into a first ground section and a second ground section by a non-conducting gap formed in the second conductor, so that the two ground sections are mutually isolated from a direct current (DC) point of view. The two ground sections are connected by one or more capacitors which effectively short-circuit the two sections from an alternating current (AC) point of view. The second ground section is bonded to a conductive optical bench, upon which the electro-optical component is positioned directly, the first electrode of the component being bonded to the optical bench. The second electrode of the electro-optical component is electrically connected to the live conductor of the micro-strip.  
           [0009]    The assembly thus enables the electro-optical component to be DC biased independently of an AC level which feeds the component. Furthermore, since the electro-optical component mates directly with the optical bench, optical alignment of the component is significantly easier than electro-optical assemblies wherein the component is not in direct contact with the optical bench.  
           [0010]    In some preferred embodiments of the present invention, the assembly is implemented as two subassemblies. A first sub-assembly comprises the transmission line implemented as described above and coupled with the one or more capacitors. A second subassembly comprises the electro-optical component mated with the optical bench. Preferably, the second subassembly is used to optically align the electro-optical component, and then the first sub-assembly is coupled to the second sub-assembly, to form the complete electro-optical assembly.  
           [0011]    There is therefore provided, according to a preferred embodiment of the present invention, an electro-optical assembly, including:  
           [0012]    an optical sub-assembly, including:  
           [0013]    an electro-optical component including an optical region and a first and a second electrode coupled thereto; and  
           [0014]    a conductive optical bench in contact with the second electrode of the electro-optical component, the optical bench being adapted to permit optical alignment of the electro-optical component while making such contact; and  
           [0015]    a transmission line including:  
           [0016]    a live conductor;  
           [0017]    a ground conductor insulated from the live conductor; and  
           [0018]    a port adapted to receive a signal, such that the live and ground conductors are coupled to the first and second electrodes of the electro-optical component so as to convey the signal between the port and the electro-optical component and to provide a direct current (DC) bias level to the electro-optical component independent of the signal.  
           [0019]    Preferably, the conductive optical bench is in direct mechanical and electrical contact with the second electrode.  
           [0020]    Preferably, the transmission line includes a micro-strip line which is adapted to operate at frequencies up to approximately 50 GHz.  
           [0021]    Preferably, the ground conductor includes a first ground section and a second ground section separated from the first ground section by an insulating gap, wherein the first and second ground sections are coupled together capacitively, and wherein the first ground section is connected to the conductive optical bench.  
           [0022]    Further preferably, the first and second ground sections are coupled by at least one capacitor so as to form an effective short-circuit between the two sections at alternating current (AC) frequencies in a range from approximately 1 kHz to approximately 50 GHz.  
           [0023]    Further preferably, the transmission line and the at least one capacitor are fabricated as an electrical subassembly, and the electro-optical assembly is fabricated by coupling the electrical sub-assembly to the optical sub-assembly.  
           [0024]    Preferably, the optical bench is adapted to permit the optical alignment after the live and ground conductors of the transmission line are coupled to the first and second electrodes of the electro-optical component.  
           [0025]    Preferably, the assembly includes circuitry which matches an impedance of the electro-optical component to the impedance of the transmission line.  
           [0026]    Further preferably, the circuitry includes a resistor connected between the first electrode and the ground conductor.  
           [0027]    Alternatively or additionally, the circuitry includes a resistor and a capacitor connected in series between the first electrode and the conductive optical bench.  
           [0028]    There is further provided, according to a preferred embodiment of the present invention, an electro-optical assembly, including:  
           [0029]    an electro-optical component including an optical region and a first and a second electrode coupled thereto;  
           [0030]    a conductive optical bench, in contact with the second electrode of the electro-optical component, the bench being adapted to permit optical alignment of the electro-optical component while making such contact;  
           [0031]    a transmission line including a live conductor and a ground conductor insulated from the live conductor, the live conductor being bonded to the first electrode of the electro-optical element, the ground conductor including a first ground section and a second ground section electrically connected to the optical bench and insulated from the first ground section by a non-conductive gap therebetween, the second ground section being capacitively coupled to the first ground section.  
           [0032]    Preferably, the conductive optical bench is in direct mechanical and electrical contact with the second electrode.  
           [0033]    Preferably, the transmission line includes a micro-strip line which is adapted to operate at frequencies up to approximately 50 GHz.  
           [0034]    Preferably, the first and second ground sections are coupled by at least one capacitor so as to form an effective short-circuit between the two sections at alternating current (AC) frequencies in a range from approximately 1 kHz to approximately 50 GHz.  
           [0035]    Preferably, the optical bench is adapted to permit the optical alignment after the live and ground conductors of the transmission line are coupled to the first and second electrodes of the electro-optical component.  
           [0036]    Preferably, the assembly includes circuitry which matches an impedance of the electro-optical component to the impedance of the transmission line.  
           [0037]    Further preferably, the circuitry includes a resistor connected between the first electrode and the first ground section.  
           [0038]    Alternatively or additionally, the circuitry includes a resistor and a capacitor connected in series between the first electrode and the conductive optical bench.  
           [0039]    There is further provided, according to a preferred embodiment of the present invention, a method for operating an electro-optical assembly, including:  
           [0040]    positioning an electro-optical component including an optical region and a first and a second electrode coupled thereto, so that the second electrode contacts a conductive optical bench;  
           [0041]    aligning the electro-optical component while maintaining the contact; and  
           [0042]    coupling a transmission line, including a live conductor and a ground conductor insulated from the live conductor and a port adapted to receive a signal, to the electro-optical component, such that the live and ground conductors are coupled to the first and second electrodes of the electro-optical component, the transmission line being adapted to convey the signal between the port and the electro-optical component and to enable a direct current (DC) bias level to be applied to the electro-optical component independent of the signal.  
           [0043]    Preferably, positioning the electro-optical component includes placing the component in direct mechanical and electrical contact with the second electrode.  
           [0044]    Preferably, the transmission line includes a micro-strip line which is adapted to operate at frequencies up to approximately 50 GHz.  
           [0045]    Preferably, the ground conductor includes a first ground section and a second ground section separated from the first ground section by an insulating gap, and wherein coupling the transmission line includes coupling the first and second ground sections capacitively and connecting the first ground section to the conductive optical bench.  
           [0046]    Further preferably, coupling the first and second ground sections capacitively includes coupling the first and second ground sections by at least one capacitor so as to form an effective short-circuit between the two sections at alternating current (AC) frequencies in a range from approximately 1 kHz to approximately 50 GHz.  
           [0047]    Preferably, the method includes:  
           [0048]    fabricating the transmission line and the at least one capacitor as an electrical sub-assembly;  
           [0049]    fabricating the electro-optical component and the conductive optical bench as an optical sub-assembly; and  
           [0050]    coupling the electrical sub-assembly to the optical sub-assembly to form the electro-optical assembly.  
           [0051]    Preferably, aligning the electro-optical component includes performing an alignment after coupling the transmission line.  
           [0052]    Preferably, aligning the electro-optical component includes adjusting an optical element to be in alignment with the electro-optical component and mechanically coupling the optical element to the conductive optical bench after performing the adjustment.  
           [0053]    Preferably, the method includes matching an impedance of the electro-optical component to the impedance of the transmission line.  
           [0054]    Further preferably, matching the impedance includes connecting a resistor between the first electrode and the ground conductor.  
           [0055]    Alternatively or additionally, matching the impedance includes connecting a resistor and a capacitor in series between the first electrode and the conductive optical bench.  
           [0056]    There is further provided, according to a preferred embodiment of the present invention, a method for operating an electro-optical assembly, including:  
           [0057]    positioning an electro-optical component, having an optical region and a first and a second electrode coupled thereto, on a conductive optical bench so that the second electrode contacts the bench;  
           [0058]    aligning the electro-optical component while the second electrode is in contact with the bench;  
           [0059]    bonding a live conductor of a transmission line to the first electrode of the electro-optical component;  
           [0060]    providing a first ground section of the transmission line for connection to a ground; and  
           [0061]    connecting a second ground section of the transmission line, which is separated by a non-conductive gap from the first ground section and is capacitively coupled to the first ground section, to the optical bench.  
           [0062]    Preferably, positioning the electro-optical component includes placing the conductive optical bench in direct mechanical and electrical contact with the second electrode.  
           [0063]    Preferably, the transmission line includes a micro-strip line which is adapted to operate at frequencies up to approximately 50 GHz.  
           [0064]    Preferably, the method includes coupling the first and second ground sections by at least one capacitor so as to form an effective short-circuit between the two sections at alternating current (AC) frequencies in a range from approximately 1 kHz to approximately 50 GHz.  
           [0065]    Preferably, aligning the electro-optical component includes performing an alignment after bonding the live conductor of the transmission line and connecting the second ground section of the transmission line.  
           [0066]    Preferably, aligning the electro-optical component includes adjusting an optical element to be in alignment with the electro-optical component and mechanically coupling the optical element to the conductive optical bench after performing the adjustment.  
           [0067]    Preferably, the method includes matching an impedance of the electro-optical component to the impedance of the transmission line.  
           [0068]    Further preferably, matching the impedance includes connecting a resistor between the first electrode and the first ground section.  
           [0069]    Alternatively or additionally, matching the impedance includes connecting a resistor and a capacitor in series between the first electrode and the conductive optical bench.  
           [0070]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0071]    [0071]FIG. 1 illustrates apparatus, as is known in the art, for electrically and optically coupling an electro-optical component; and  
         [0072]    [0072]FIG. 2 is a schematic diagram of an assembly for biasing an electro-optical component, according to a preferred embodiment of the present invention; and  
         [0073]    [0073]FIG. 3 is a schematic diagram of an alternative assembly for biasing the electro-optical component, according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0074]    Reference is now made to FIG. 2, which is a schematic diagram of an assembly  50  for biasing an electro-optical component, according to a preferred embodiment of the present invention. A transmission line  56  feeds an electro-optical component  76 . Component  76  comprises an upper electrode  70  and a lower electrode  74 . Component  76  also comprises an optical region  71 , coupled to the electrodes, which, in order to function correctly, requires optical alignment. Component  76  is typically an electro-absorption modulator (EAM) or a photo-diode detector (PDD). However, it will be understood that electro-optical component  76  comprises substantially any component having two electrodes and which acts as a transducer converting between electrical energy and optical radiation energy, or which utilizes electrical energy to change an optical characteristic of the component, such as a change of a refractive index of the component. Component  76  requires alignment with an optical element  68 , herein assumed by way of example to comprise a fiber optic. Assembly  50  is preferably implemented to operate at frequencies in a range from approximately 1 kHz to approximately 50 GHz, although it will be understood that preferred embodiments of the present invention may operate at frequencies different from this frequency range.  
         [0075]    Transmission line  56 , most preferably a micro-strip transmission line, comprises an upper “live” conductor  54  and lower ground conductors  52  and  65 , and is preferably implemented from specialized material, such as double-sided 10 mil alumina substrate, although any other material known in the art for implementing transmission lines at a frequency of operation of component  76  may be used to implement line  56 . In the specification and in the claims, the terms micro-strip transmission line and micro-strip line are assumed to refer to a transmission line having a first conductor and a second conductor, one of the conductors acting as a substantially infinite ground plane, the two conductors not lying in the same plane. A non-conductive gap  62  is formed on the lower surface of line  56 , thus breaking the ground conductor of the micro-strip into ground sections  52  and  65 . Ground sections  52  and  65  act as a ground plane. Preferably, electrical signals to line  56  are input to a port  55  of the line, and the line then conveys the signals to component  76 . Alternatively or additionally, port  55  acts as an output port, outputting signals received from component  76  via line  56 .  
         [0076]    A first electrode of a first capacitor  60 , which has a capacitance of the order of 1 nF, is bonded to section  52 . Capacitor  60  is typically disk-shaped. A second capacitor  58 , typically a surface mounted component having a capacitance of the order of 1 μF, is connected in parallel with the first capacitor. The parallel capacitors act as a capacitor providing a broad-band short-circuit at AC frequencies in a range from approximately 1 kHz to approximately 50 GHz. A conductor  66 , typically a gold wire or ribbon, is bonded to a second electrode of capacitor  60  (and of capacitor  58 ) and to section  65 . Thus, ground section  65  is effectively AC coupled to ground section  52 , but is DC insulated from ground section  52 , and so “floats” with reference to section  52 .  
         [0077]    Ground section  65  is bonded to a conductive optical bench  72 , upon which is mounted electro-optical component  76 . Lower electrode  74  of component  76  mates with bench  72 , and is bonded to the bench. Upper electrode  70  of component  76  is connected to upper conductor  54  of line  56 . The connection between electrode  70  and conductor  54  is implemented by bonding a conductor  64 , preferably a gold wire or ribbon, between the electrode and the conductor. A resistor  73  is most preferably connected between ground section  52  and electrode  70 , the resistor acting as an impedance match and as a DC path to ground.  
         [0078]    Alternatively, for example, where there is a DC component on conductor  54  such as is generated from an Electro-Absorption Modulator (EAM) driver, resistor  73  is kept floating. Such a system is described in more detail below with respect to FIG. 3.  
         [0079]    Element  68  and component  76  need to be aligned to extremely close tolerances, of the order of 0.2 μm, in order for assembly  50  to function efficiently. Furthermore, the alignment needs to be maintained during operation of assembly  50 , when ambient parameters such as temperature may vary significantly. To accomplish these aims, bench  72  is manufactured to sufficiently close tolerances so that component  76 , when mounted directly on the bench as described above, is approximately optically aligned with element  68 . More exact alignment of element  68  with component  76  may then be performed by positioning element  68 , and by mechanically coupling element  68  to the bench when alignment is achieved.  
         [0080]    It will be appreciated that initial approximate alignment of component  76 , subsequent exact alignment of the component, and maintenance of the alignment during operation of assembly  50 , are all facilitated by mounting the component directly on bench  72 , so that the component and the element aligned with the component are close to the bench. Because of the direct connection between component  76  and optical bench  72 , ambient parameter changes, such as ambient temperature changes, have substantially no effect on the alignment of component  76 .  
         [0081]    (In the system described with reference to FIG. 1, where electro-optical component  10  is mounted on capacitor  24 , initial and subsequent alignment of component  10  are difficult because of the distance of the component from bench  26 . Furthermore, any ambient parameter changes during operation of the system of FIG. 1 significantly affect the alignment by causing movement or expansion or contraction of capacitor  24 .)  
         [0082]    In some preferred embodiments of the present invention, assembly  50  is implemented as two separate sub-assemblies  80  and  82 . First sub-assembly  80  comprises transmission line  56 , capacitors  58  and  60 , resistor  73 , and conductor  66 . The elements of sub-assembly  80  are coupled together substantially as described above. Second sub-assembly  82  comprises electro-optical component  76  mounted and bonded, as described above, to optical bench  72 . Typically, sub-assembly  82  is mounted in a receiving package for assembly  50 , and exact alignment of element  68  with electro-optical component  76  is performed substantially as described above. Sub-assembly  80  is then coupled to sub-assembly  82  by bonding ground section  65  to the optical bench, and by bonding conductor  64  to conductor  54  and electrode  70  of electro-optical component  76 .  
         [0083]    It will be appreciated that the arrangement of assembly  50  isolates electrode  74  from ground section  52  from a DC point of view, while the electrode is coupled to the ground section from an AC point of view. Thus, component  76  may be DC biased by applying DC levels to electrodes  70  and  74  independent of any AC transmission injected into transmission line  56 . Ground section  52  is unaffected by the application of DC to electrode  74 , since the latter is DC insulated from the ground section by gap  62 . Furthermore, since electro-optical component  76  mates directly with optical bench  72 , alignment of the optical bench effectively approximately pre-aligns the component, and exact alignment with optical element  68  by subsequent adjustment of the bench is straightforward, unlike the prior art assembly described with reference to FIG. 1.  
         [0084]    [0084]FIG. 3 is a schematic diagram of an assembly  90  for biasing an electro-optical component, according to an alternative preferred embodiment of the present invention. Apart from the differences described below, the operation of assembly  90  is generally similar to that of assembly  50  (FIG. 2), so that elements indicated by the same reference numerals in both assemblies  50  and  90  are generally identical in construction and in operation. Assembly  90  is preferably used when there is a DC component present on port  55 . A first electrode of a capacitor  94 , which has a capacitance of the order of 1 nF, is bonded to optical bench  72 . Capacitor  94  is typically disk-shaped. A second capacitor  96 , typically a surface mounted component having a capacitance of the order of 1 μF, is connected in parallel with the first capacitor.  
         [0085]    A resistor  92 , replacing resistor  73  of assembly  50 , is connected between a second electrode of capacitor  94  and electrode  70  of component  76 . The parallel capacitors act as a capacitor providing a broad-band short-circuit at AC frequencies in a range from approximately 1 kHz to approximately 50 GHz. However, unlike assembly  50 , the resistor of assembly  90  is floating and does not provide a DC path to ground. It will be understood that resistor  92  and capacitors  94  and  96  act as circuitry matching an impedance of electro-optical component  76  to transmission line  56 . In assembly  50 , resistor  73  performs a substantially similar function.  
         [0086]    It will be appreciated that assembly  90  may be implemented as sub-assemblies, substantially as described for assembly  50 , based on transmission line  56  and optical bench  72 .  
         [0087]    It will be further appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.