Abstract:
Light emitting elements with preselected or adjustable impedance characteristics are provided. Embodiments using a preselected impedance characteristic obtain significant performance benefits compared to the prior art. Embodiments having an adjustable impedance may alter the impedance associated with the light emitting component such that it has a substantially constant resistive or reactive impedance that improves certain performance attributes. This solution virtually eliminates the need for external compensation components and relieves the burden of impedance matching and circuit specialization from the driver circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to light emitting elements, and more particularly, to improving the performance characteristics of light emitting elements.  
         [0002]     Light emitting elements such as light emitting diodes (LEDs), laser diodes, and vertical cavity surface emitting lasers (VCSELs) are used today in a wide variety of electronic applications. Common examples of products that employ light emitting elements include DVD players and high speed optical communications systems. Frequently, the role of light emitting elements involves the generation of light pulses for data acquisition or transmission. To function in this capacity, driver circuitry is typically utilized to supply the appropriate electrical differential across the light emitting element to generate digital optical signals.  
         [0003]     Currently, many light emitting elements are fabricated with varying impedance values. A VCSEL, for example, typically has an impedance value that varies between about 17-50 Ohms. This variation poses problems for driver circuitry because of the likely impedance mismatch that results between the output of the driver circuitry and the input of the VCSEL. Foremost among these problems are the signal reflections that occur along the transmission line between the driver circuit and the VCSEL that reduce signal quality and limit response time.  
         [0004]     Presently, this problem is dealt with by manipulating the operating characteristics of the driver circuitry. One popular solution, for example, includes the addition of external termination components that absorb reflected energy. This solution is not optimal because it requires system manufacturers to determine the impedance of each light emitting element individually and couple the appropriate compensation components to the driver circuitry. Other popular solutions include the use of precision driver modules that can accommodate an impedance mismatch without losing significant performance. Such systems, however, inevitably sacrifice certain amounts of bandwidth and frequency response.  
         [0005]     Thus, in view of the foregoing, it would be desirable to provide circuits and methods that reduce or minimize the impedance variation associated with light emitting elements.  
         [0006]     It would also be desirable to provide circuits and methods that enable adjustment of the transfer function associated with light emitting elements to improve or optimize certain performance attributes.  
       SUMMARY OF THE INVENTION  
       [0007]     Thus, in view of the foregoing, it is an object of the present invention to provide circuits and methods that reduce or minimize the impedance variation associated with light emitting elements.  
         [0008]     It is also an object of the present invention to provide circuits and methods that enable adjustment of the transfer function associated with light emitting elements to improve or optimize certain performance attributes.  
         [0009]     These and other objects are accomplished in accordance with the principles of the present invention by providing light emitting elements with additional circuitry. The additional circuitry may have a preselected or adjustable impedance characteristic. Embodiments using a preselected impedance characteristic obtain significant performance benefits compared to the prior art. Embodiments having an adjustable impedance may alter the impedance associated with the light emitting component such that it has a substantially constant resistive or reactive impedance that improves certain performance attributes. This solution virtually eliminates the need for external compensation components and relieves the burden of impedance matching and circuit specialization from the driver circuit.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:  
         [0011]      FIG. 1A  is an illustrative physical layout of a light emitting component constructed in accordance with the principles of the present invention;  
         [0012]      FIG. 1B  is a schematic diagram of additional circuitry that may be included in the light emitting component of  FIG. 1 ;  
         [0013]      FIG. 2  is a schematic diagram of the physical layout shown in  FIG. 1 ;  
         [0014]      FIG. 3  is an illustrative physical layout of another light emitting component constructed in accordance with the principles of the present invention;  
         [0015]      FIG. 4  is a schematic diagram of the equivalent circuit associated with the physical layout shown in  FIG. 3 ;  
         [0016]      FIG. 5  is a schematic diagram of a light emitting component constructed in accordance with the principles of the present invention coupled to driver circuitry through a transmission line.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  is an illustrative physical layout of a light emitting component  100  constructed in accordance with the principles of the present invention. Component  100  may include a bond pad  110 , an adjustable resistor  120 , and a light emitting element  130 . Although component  100  is depicted as a “chip-on-board” type package, any other suitable packages types such a dual inline packages (DIPs) and “can” packages may be used if desired.  
         [0018]     Light emitting element  130  may be any circuit element suitable for generating light pulses such as an LED, a laser diode, a VCSEL, etc. Adjustable resistor  120  may be created from any material suitable for forming resistive elements on an a substrate such as a metal resistor formed from the same material as bond pad  110  or a thin-film or thick-film type resistor.  
         [0019]     In operation, driver circuitry (not shown) is coupled to light emitting element  130  via bond pad  110 , adjustable resistor  120 , and signal paths  115  and  125 . Signals from the driver circuitry pass through resistor  120  and into light emitting element  130  turning it ON and OFF to produce light pulses. A schematic representation of component  100  is shown in  FIG. 2 .  
         [0020]     The purpose of adjustable resistor  120  is to normalize the impedance variation associated with light emitting element  130  so the equivalent input impedance of light emitting component  100  is a substantially constant predetermined value. For example, if a given light emitting element has an impedance of 29 Ohms, and another light emitting element manufactured from the same process has an impedance of 34 Ohms, resistors  120  may be adjusted to 21 Ohms and 16 Ohms, respectively, so that both components have an input impedance of 50 Ohms. This configuration allows circuit manufacturers to produce light emitting components having a predetermined substantially constant impedance and thus virtually eliminate the need for compensation components or other specialized circuit drivers.  
         [0021]     Determining the proper value of adjustable resistor  120  may be accomplished in several ways. One method includes manufacturing component  100  and measuring the resistance of light emitting element  130 . Next, the value of resistor  120  is adjusted (e.g., trimmed) so that the total impedance of component  100  as seen from bond pad  110  is set at a desired value (e.g., 50 Ohms).  
         [0022]     Another way of adjusting resistor  120  includes measuring the resistance of component  100  from bond pad  110  and adjusting resistor  120  until the total impedance of component  100  as seen from bond pad  110  reaches the desired value.  
         [0023]     In some instances, it may be desirable to adjust the value of resistor  120  while monitoring the optical response of component  100 . This may be done to obtain a particular optical response from component  100  (e.g., to optimize an “eye opening”) irrespective of input impedance.  
         [0024]     In an alternate embodiment of component  100 , resistor  120  may be fabricated to a nominal fixed value subject to variations associated with resistor fabrication (e.g., within about ±20% of the intended value). In this case, resistor  120  is not adjusted, and its function is merely to bring the total impedance of light emitting component  100  to a more desirable value. For example, assume the average impedance of a particular group of light emitting elements  110  is known to be 31.6 Ohms and to lie between 20 Ohms and 50 Ohms in all cases. In a commonly encountered 50 Ohm transmission line environment, a light emitting component would suffer from reflections of 43% at a minimum value of 20 Ohms. With this in mind, a manufacturer may construct resistor  120  such that its nominal value is also 18.4 Ohms subject to manufacturing tolerances so the final impedance of component  100  is about 50 Ohms. With this arrangement, the worst case reflection would be reduced by a factor of 2, or down to about 23%.  
         [0025]     It will be appreciated from the above that the impedance of component  100  may be set to one of numerous values as specified, for example, by customer requirements. In most cases, the only restriction on this feature is the minimum impedance of light emitting element  130 . Thus, if desired, the equivalent impedance of light emitting component  100  may be increased to values significantly greater than the original impedance value of light emitting element  100 .  
         [0026]     Furthermore, in an another embodiment of the present invention, a “template” version of component  100  may be provided with little or no adjustment of resistor  120 . In this case, a user, such as a component manufacturer, may adjust resistor  120  to meet specific needs. For example, a template version of component  100  may be coupled to a specific driver circuit. Next, resistor  120  may be adjusted to attain a substantially perfect impedance match between the driver circuit and component  100  to optimize system performance. This type of “component customization” makes component  100  suitable for use in a wide variety of high performance applications such as those that involve coding schemes with high overheads or require fast data transfer rates with limited error tolerance.  
         [0027]     It will be understood that resistor  120  may be adjusted during various stages of fabrication of component  100  including adjustment of the final product. For example, resistor  120  may be adjusted at the wafer level, during construction of the optical subassembly (e.g., when an optical connector is attached and aligned with element  130 ), during module assembly (e.g., the addition of driver circuitry and component packaging), or by an end item user.  
         [0028]     Moreover, additional network components may be added to light emitting component  100  to adjust the current threshold (i.e., the magnitude of a drive signal required to produce a desired minimum light level) and the slope efficiency (i.e., the incremental response of the optical output signal of element  110  compared to the drive signal when the drive signal is above the threshold level) of light emitting element  130 .  
         [0029]     For example, as shown in  FIG. 1B , adjustable resistors  121  and  122  and light emitting element  131  may be added to component  100 , if desired. With this embodiment, light emitting element  131  acts as a voltage clamp to bias light emitting element  130 . The optical output produced by element  131  is not necessarily used. Resistor  121  may be adjusted to divert a portion of an applied drive signal away from element  130  and into element  131 , thereby reducing the optical output of element  130 . With this capability, it is possible to vary the value of resistor  121  so that the optical output of element  130  reaches a predetermined level for a given value of drive current. This allows the selection of predefined and substantially constant electrical-to-optical conversion parameters for component  100  despite the large variations in efficiency commonly associated with element  130 .  
         [0030]     In addition, the current threshold at which light emitting element  130  emits light may be set to a predetermined value by adjusting variable resistor  122 .  
         [0031]     It will be appreciated that the features of the present invention shown in  FIGS. 1A and 1B  may be used separately, together, or in various combinations depending upon the desired functionality. For example, component  100  may be constructed such that it includes only light emitting element  130  and variable resistor  122 . In this embodiment, component  100  would have a preset current threshold but not a preset input impedance or a normalized electrical-to-optical conversion efficiency. All combinations and permutations of these features shall be recognized as within the scope of the present invention.  
         [0032]      FIG. 3  is an illustrative physical layout of a light emitting component  200  constructed in accordance with the principles of the present invention. Component  200  is similar to component  100  and further includes adjustable capacitor  140  and adjustable inductor  150 . Although component  200  is depicted as a “chip-on-board” type package, any other suitable packages types, such as a dual inline packages (DIPs) and “can” packages may be used if desired. Component  200  may also employ some or all of the circuit elements shown in  FIG. 1B .  
         [0033]     The primary purpose of adjustable capacitor  140  and adjustable inductor  150  is to permit variation of the transfer function associated with light emitting component  200 . With this arrangement, circuit manufacturers may produce light emitting components with specialized transfer functions that improve or optimize certain performance characteristics. For example, by adjusting the values of some or all of the circuit elements on component  200 , it is possible to obtain a substantially resistive characteristic impedance (e.g., by “tuning out” or compensating for package and coupling reactance).  
         [0034]     The arrangement of  FIG. 3  may also be used to create transfer functions that improve or optimize other performance characteristics such as high frequency response, time domain response, transition time, settling time, bandwidth, jitter, etc. Such optimization may include adjusting some or all of the circuit elements on component  200  to vary the shape of an optical pulse produced by element  130  to conform with a certain “mask” or to be compliant with a particular communications standard (e.g., SONET).  
         [0035]     In an alternate embodiment of component  200 , capacitor  140  and inductor  150  may be fabricated to nominal fixed values subject to variations associated with component fabrication (e.g., within about ±20% of the intended value). In this case, these components are not adjusted, and their function is merely to improve one or more of the above-mentioned performance characteristics of component  200 .  
         [0036]      FIG. 4  shows an equivalent circuit of the physical layout shown in  FIG. 3 . In  FIG. 4 , inductor  180  represents the inductance associated with a bond wire that connects to bond pad  110  (not shown), and capacitor  170  represents the capacitance associated with bond pad  110 . Light emitting element  130  is schematically represented as a laser diode.  
         [0037]      FIG. 5  shows a driver circuit  210  coupled to light emitting component  200  through a transmission line  220 . In this embodiment, the value of resistor  120  may be adjusted to normalize the impedance of light emitting element  130  to a value matching the characteristic impedance of transmission line  220  for proper line termination. The values of adjustable capacitor  140  and adjustable inductor  150  may be set to compensate for transients associated with packaging reactance. In other embodiments, driver circuit  210  may be coupled directly to component  200  using “chip-to-chip wire bonding” with no intervening transmission line (not shown). In this case, the value of bond wire inductance  180  may be somewhat greater than that associated with arrangement in  FIG. 5 .  
         [0038]     It will be appreciated, that, in many cases circuit elements  120 ,  140 , and  150  may be disposed on light emitting components  100  and  200  without deviating from normal fabrication techniques. For example, inductors, capacitors, and resistors may all be formed from the metal used to make bond pads and circuit traces.  
         [0039]     A spiral inductor, for example, having a value of approximately 1.2 nH may be formed by creating three concentric metal spirals having an outermost diameter of about 100 μm using 5 μm wide metal spaced apart about 1 μm. This specific implementation is merely exemplary and many other inductor configurations are possible. The inductance value may be increased or decreased as desired by changing the diameter of the spiral, the width of the metal, and the spacing between the portions of the metal used to form the inductor. Further, the inductor may be generally arranged as shown in  FIG. 3  to avoid overlapping with other metal traces.  
         [0040]     Capacitors may be formed by depositing a metal plate on top of the die used to form the light emitting component. The resulting capacitance depends on the size of the metal plate and the process used to create the die.  
         [0041]     Various other known integrated circuit processing techniques may be used if desired to enhance the performance of these passive circuit elements. For example, masking, plating, and etching techniques may be used to thicken inductor metal or thin resistor metal to lower resistance. A high resistance base metal such as SiCr, NiCr, or tungsten may be used to form resistors. Thin or thick film conductive layers may be added to the die to form physically smaller resistors having large resistance values with reduced parasitic capacitance to the substrate. Thin or thick film dielectric layers may be added to form parallel plate capacitors or to allow trace crossovers. Diffused or ion implanted resistors, junction capacitors, and conductors may be created using photo lithographical techniques, etc.  
         [0042]     It will be further appreciated that various methods may be used to adjust the values of the above-described components. For example, trimming may be done with a laser or abrasive on capacitor  140  or resistor  120 . Such trimming may be performed while measuring the resistance, capacitance, or other parameters of light emitting components  100  and  200 . Resistor  120 , capacitor  140 , and inductor  150  may be trimmed with a continuous adjustment, or by cutting constituent segments or tabs in discrete steps. Any such suitable method may be used. In certain embodiments, component adjustment may be done economically at the wafer level before the wafer is cut into individual components.  
         [0043]     Persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.