Patent Publication Number: US-7720386-B2

Title: Impedance matching circuit with simultaneous shielding of parasitic effects for transceiver modules

Description:
RELATED APPLICATION INFORMATION 
   This application is a Continuation application of U.S. patent application Ser. No. 10/727,817 filed on Dec. 4, 2003 now U.S. Pat. No. 7,412,172, pending, and is related to Continuation application of U.S. patent application Ser. No. 11/875,332, filed on Oct. 19, 2007, pending, both incorporated herein by reference in their entirety. 

   BACKGROUND 
   1. Technical Field 
   Aspects of the present invention relate to optical transceiver modules, and more particularly to a wideband impedance matching circuit design that simultaneously provides a high bandwidth signal path from a laser diode driver to a laser diode and shields against parasitic effects from a DC current supply of the laser diode. 
   2. Description of the Related Art 
   Laser diodes operating in high data rate optical transceivers or transmitters need a wideband high-frequency (AC or RF) connection to the laser diode driver circuit which sends the data to be transmitted, and a DC current supply connection to the laser diode for establishing the operating point. 
   Some optical modules establish the operating point through RF lines, which require higher biasing voltage and produce higher power dissipation. 
   For the optical modules where DC and RF are connected using different circuits the following considerations may apply. The RF lines demand a careful design over the bandwidth of the signal to be transmitted (here up to 15 GHz for a 10 Gb/s application). The DC connection is less sensitive, but presents additional challenges for the RF design since it must be decoupled for the entire frequency range (e.g., above 10 MHz or in general 1×10 −4  of the data rate). 
   This decoupling is done by so-called RF chokes which are commercially available. RF chokes represent a low impedance path for DC currents but a high impedance path for RF signals. However, parasitic electromagnetic effects (capacitive coupling and inductive voltage drops) associated with the placement and design of RF chokes can disable the decoupling and hence degrade signal transmission to the laser diode. 
   SUMMARY 
   Parasitic capacitance to ground of RF choke pads (C g ) together with parasitic inductance (L b ) of bonding wires, which connect to the laser, form a resonant circuit which creates a parallel (shunt) low impedance path to high-frequency signals at its resonance frequency. The resonance frequency can be approximated by: 
             f   res     =     1     2   ⁢   π   ⁢         L   b     *     C   g                   
Typical values for f res  can be as low as several GHz.
 
   Increasing the resonance frequency f res  by lowering the values of the parasitic elements C g  and L b  is only possible up to a certain limit. The size of the RF chokes, the minimum bonding wire distance and other design or manufacturing constraints usually set a lower limit to the parasitic inductance and capacitance values. Hence, the full operating bandwidth of the device can usually not be made free of any parasitic RF choke resonances. 
   It may be noted that not only bonding wires, but any electrical connection between laser diode and RF choke will have a parasitic inductance. As a consequence, the considerations outlined above apply to different electrical connections as well, e.g. ribbon wires or direct attachment. 
   Therefore, a transceiver or other RF device includes a signal layer disposed over a ground plane having radio frequency (RF) transmission lines configured and dimensioned to provide impedance matching along the RF lines. A shield is formed as a part of the RF lines and disposed below an RF choke of a DC current supply to form an intermediate capacitance between the choke and the shield to control parasitic effects. 
   A method for fabricating a transceiver, which simultaneously provides impedance matched transmission for radio frequency (RF) and shields against transmission losses due to parasitic effects, includes the steps of identifying parasitic electromagnetic elements associated with an RF choke for a given placement on a transmission path, and placing and dimensioning the RF lines on the transmission path to form impedance matched RF lines wherein a portion of the RF lines shield the RF choke for a given bandwidth such that impedance matching and control of parasitic effects of the RF choke are simultaneously provided. 
   These and other objects, features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
       FIG. 1  is a top view of an illustrative transceiver in accordance with one illustrative embodiment; 
       FIG. 2  is a cross-sectional view taken at section line  2 - 2  of  FIG. 1 ; 
       FIG. 3A  is a top view showing a signal layer with impedance matched RF lines, which form a shield in accordance with an illustrative embodiment; 
       FIG. 3B  is a top view showing a DC current supply path with RF choke elements and further showing areas of potential parasitic elements; 
       FIG. 4  is a schematic diagram showing parasitic elements including a shield capacitor formed in accordance with an aspect of the present invention; 
       FIG. 5  is an S-parameter plot in dB versus frequency for three transmission cases of the equivalent circuit in  FIG. 4 ; 
       FIG. 6  is an S-parameter plot in dB versus frequency for simulated transmission of a prototype design; 
       FIG. 7  is an S-parameter plot in dB versus frequency for measured transmission of a prototype design; and 
       FIG. 8  is a block/flow diagram for fabricating a transceiver, which simultaneously provides impedance-matched transmission for radio frequency (RF) and shields against transmission losses due to parasitic effects. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Systems and methods for wideband impedance matching and circuit designs are provided for optical transceiver modules that simultaneously provide a high bandwidth signal path to the laser diode and a shield against parasitic effects from the DC current supply of the laser diode. Aspects of the present invention are based on a co-design of the electrical high-frequency signal lines and the DC current supply lines. One embodiment utilizes a specific implementation of a Wheatstone bridge where the shielding is acting as a bypass (speed-up) capacitor. 
   Parasitic effects may occur whenever there is a metallic plane, also known as a ground plane, underneath an optical transceiver module circuit. In accordance with one embodiment, an impedance matching circuit presented herein is implemented on silicon. However, the substrate material may include other materials, for example, ceramics, organics, printed or flexible circuit boards, etc. Also, the DC current supply does not have to be on a separate submount or of the shape and size presented hereinbelow. 
   Referring now in detail to the figures in which like numerals represent the same or similar elements and initially to  FIG. 1 , a transceiver assembly or other device  10  includes an optical bench or substrate  11  and is shown in accordance with one embodiment. Assembly  10  includes RF chokes  12  placed on submounts  14 , e.g., small pieces of dielectric material, with suitable metallic pads  20  for round or ribbon bond wiring. The submount size is preferably kept as small as possible to permit a placement close to a laser diode  16 . The submount height and material are preferably chosen for an appropriate manufacturing process and easy attachment to the optical bench  11 . 
   Location and/or modification of the RF choke submount  14  are arranged such that parasitic capacitance can be shielded and hence reduced, by the RF line design. This means metallic contacts  20  of the submount  14  are placed over RF lines  22  and as close as possible to the laser diode  16 . 
   Identification of the parasitic electromagnetic elements associated with the RF chokes  12  and their placement in the design is performed and is used to influence the placement and design of the impedance matching circuitry. The parasitic elements include a parasitic inductance of the electrical path from choke  12  to laser  16 , for example wires  21 , and a parasitic capacitance including the RF choke terminals  13  and a ground plane (not shown) of the optical bench  10  (both the RF choke terminals  13  and any metallic pad  20  that they are located on contribute to this capacitance). Bonding wires  23  connect the RF chokes to the DC power supply (not shown). 
   The parasitic capacitance influences the design of the impedance matching circuit  22  such that a portion of the circuit shields the RF choke  12  over a desired bandwidth. The design of circuit may be iteratively adjusted to meet the desired specifications to achieve sufficient shielding of the parasitic capacitance by the RF lines  22 . 
   Assembly  10  does not try to directly reduce the parasitic elements; instead it shields the RF choke elements  12  that contribute to the parasitic capacitance to ground, which effectively eliminates a resonant circuit, which would otherwise cause the parasitic effects. 
   A photodiode  30  is connected to control circuitry through connections  32 . Connections  32  are bonded or otherwise connected to traces  36  (external to bench  11 ). Similarly, connections  38  connect RF lines  22  to traces  40  formed off optical bench  11 . DC blocking capacitors can be provided before connections  38  to prevent DC biasing of the AC source, e.g. the laser diode driver. A laser diode  16  is placed in a gap (d) between RF lines  22 . A lens  44 , such as a ball lens is mounted adjacent to laser  16  for focusing the output signal into an isolator  46  or optical fiber (not shown). 
   In one embodiment, bench  11  is formed from high resistivity silicon (e.g., about 1 kOhm-cm). Ceramic or organic substrates may also be employed,  FIG. 2  shows a cross-section of assembly  10  taken through a section line  2 - 2  in  FIG. 1 . 
   Referring to  FIG. 2 , in the embodiment shown, a silicon bench  11  is mounted on a conductive block  50 . Block  50  is preferably formed from a highly conductive material such as a metal, and particularly, copper or its alloys. Other suitable materials may include gold, silver or other metals and their alloys. Block  50  is maintained at ground potential or functions as an AC ground reference for bench  11 . Submount  14  is mounted on bench  11 . Bench  11  includes RF lines  22  and forms the signal layer. RF choke  12  is mounted on submount  14 . A choke terminal  13  and pad  20  on submount  14  form capacitances both to the signal layer (C s ) and the ground layer (C g ), the latter being parasitic. By simultaneously balancing these capacitances, the advantages of aspects of the present invention are realized. Namely, impedance matched RF signal transmission is achieved from a laser diode driver (not shown) to laser diode and a DC biasing of the laser diode  16  without any detrimental effect on the RF signal transmission. This is achieved through the design of the complex shape of RF lines  22 . 
   In particular, RF lines  22  include dimensions which provide a matched impedance for incoming RF signals, yet prevent such signals from being lost due to parasitics in the DC power connections as in prior art systems. 
   To further illustrate the advantages of one embodiment,  FIG. 3A  schematically shows features of RF lines  22  which provide impedance matching and shielding to counter parasitic inductance and capacitance. 
   Referring to  FIG. 3A , a signal layer includes RF lines  22 . RF lines  22  are geometrically configured to provide an impedance-matched network for an AC signal path. In addition, a portion  52  of line  22  provides shielding to reduce C g  by introducing a capacitance C s  between pad  20  of submount  14  and the signal layer (e.g., RF lines  22 ) as illustratively shown in  FIG. 3B . Note that  FIG. 3B , eliminates signal layer features for illustrative purposes. By its geometric dimensions, lines  22  are determined to permit AC power to a laser without power loss due to parasitics of the DC supply path over a range of acceptable operation frequencies (e.g., 2-20 GHz). 
   Referring to  FIG. 4 , an equivalent circuit is illustratively shown to demonstrate advantages of preferred embodiments. Providing a shield ( 52  in  FIG. 3A ) introduces an intermediate capacitance (C s ). R m  represents a resistor that provides a match of a low impedance laser diode R l , to a laser diode driver (Driver), which may include, for example, a thin film resistor  54  ( FIG. 3A ). C g  is the parasitic capacitance of the RF choke to ground and L b  is the parasitic inductance of the connection between laser diode ( 16 ) and RF choke ( 12 ). The RF choke impedance is assumed to be much higher than both R m  and R l  for high frequencies and is therefore not considered here for simplicity. “Wideband-Match” is a modeled impedance which accounts for the geometric effects of RF lines  22 . 
   In the unshielded case, i.e., without C s , C g  and L b  form a resonant circuit, which offers a parallel low impedance path to the high-frequency signals at its resonance frequency. This leads to a loss of transmitted energy. Advantageously, introduction of the shield  52  ( FIG. 3A ) will have at least two effects. First, C g  will be reduced, and second, a bypass (speed-up) capacitance C s  across the matching resistor R m  is introduced. By doing so a complex Wheatstone Bridge like circuit is created with R m  and R l  being the one branch and C s  and C g  being the other. 
   The following cases can now be distinguished: 
   1) C s /C g &lt;R l /R m  Shielding is below a target value. There will be a loss in transmission. 
   2) C s /C g =R l /R m  The shielding is exactly on target (L b  is without current). Transmission is flat. 
   3) C s /C g &gt;R l /R m  Shielding is above target. There is a peak in transmission. 
   Referring to  FIG. 5 , equivalent circuit simulations were performed for all three of the above cases. Values were selected to provide distinction for the three cases. Magnitudes for parameters in the illustration include: Driver=50 Ohm, R m =40 Ohm, R l =10 Ohm and L b =10 nH. 
   In case 1, no shielding, C s =0, C g =40 fF and C s /C g =0. Plot  101  shows a notch in transmission on a graph of transmission (dB) versus frequency (GHz). 
   In case 2, shielding on target, C s =10 fF, C g =40 fF and C s /C g =R l /R m . Plot  102  shows a flat response in transmission on a graph of transmission (dB) versus frequency (GHz). 
   In case 3, complete shielding, above target C s =10 fF, C g =0 fF and C s /C g =infinite. Plot  103  shows a peak response in transmission on a graph of transmission (dB) versus frequency (GHz). 
   The shielding, in accordance with aspects of this disclosure, is accomplished by an integral part of the design, namely the RF lines  22  on the silicon optical bench  11  where the laser is located. The RF lines  22  and the RF choke submount  14  are designed and placed such that the shielding is exactly on or above target (e.g., C s /C g =R l /R m ) while impedance match is preserved, and impedance match is a function of the physical geometry of RF lines  22 . 
   Simulation and Measurement of Prototype 
   Referring again to  FIG. 1 , a prototype of one illustrative embodiment was constructed and tested. The prototype included the features of  FIG. 1  with the following features (Dimensions in microns): 
                                              Silicon Optical Bench (11)   5,100 × 5,100 × (about 700               high)           Laser-Diode (L) (16)   200 × 300           RF Choke (12)/Terminals (13)   1,524 × 762/380 × 762           RF lines (22)(widths)   1,250, 1,550, 400, 1,300           Resistors (R) (54)   600 × 246           Submounts (14)   2,050 × 1,600 × (about 500               high)           Pads on submounts (20)   525 × 1,400                        
Distance designated by letters in microns:
 
                                              a. center axis to Photo-Diode (PD) pad   2,100           b. RF line to RF line (min.)   400           c. RF line to RF line (max.)   1,200           d. space for laser (L)   308           e. Y-branch length   800           f. Y-branch to PD   200           g. PD to L   500           h. L to ball lens   200           i. Ball lens to isolator   615           j. Impedance matched RF line length   600 + 650           k. Overall RF line length   2,650                        
The prototype included a silicon optical bench (about 700 microns in thickness) with a metallic ground plane ( FIG. 2 ) on the bottom, RF lines  22  on the top, and a submount  14  holding the RF chokes  12 . RF lines  22  and submount pads  20  have been co-designed to optimize shielding. The RF chokes  12  used were a 0603 standard size having a minimum impedance of 200 Ohm out to 8 GHz.
 
   Referring to  FIGS. 6 and 7 , the overall shielding effect realized in the prototype with the features listed above was simulated to be acceptable up to 15 GHz ( FIG. 6 ). The prototype with the RF choke submounts (curve  202  of  FIG. 6 ) performs similar or better than the one without RF chokes (curve  201  of  FIG. 6 ) up to 15 GHz. Simulations were done by using a 3D frequency-domain full-wave electromagnetic solver where the RF choke component was modeled as an infinite impedance. 
   Measurements ( FIG. 7 ) show that the prototype with the RF choke submounts (curve  302 ) performs similar to the one without RF chokes (curve  301 ) up to 11 GHz. Above 11 GHz a continuous decrease in transmission can be noticed; however, there is no notch in transmission. 
   Referring to  FIG. 8 , a method for fabricating a device, which simultaneously provides an impedance matched transmission line for RF and shields against parasitic effects, is illustratively shown. In block  402 , RF chokes may be placed on submounts. Submounts include pieces of dielectric material, with suitable metallic pads for bond or ribbon wiring. The submount size is preferably held to as small a size as possible to permit placement close to the laser diode, e.g., within a few hundred microns. The submount height and material should be chosen for the appropriate manufacturing process and easy attachment to the optical bench. 
   In block  404 , identification of the parasitic electromagnetic elements associated with the RF chokes and their placement in the design is determined. These elements may include a parasitic inductance including the electrical path from choke to laser (for example bonding wires) and a parasitic capacitance including the RF choke contacts and the ground plane of the optical bench (both the RF choke terminals and any metallic pad which they are located on contribute to this capacitance). The parasitic capacitance will be affected in the next step. 
   In block  406 , location and/or modification of the RF choke submount is performed such that the parasitic capacitance can be shielded, i.e. reduced, by the RF line. This may include locating the metallic contacts of the submount over the RF lines and as close as possible to the laser diode. 
   In block  408 , an impedance matching circuit (RF lines) is designed (e.g., including the placement and dimensioning of the RF lines) such that the RF lines shield the RF choke over a desired bandwidth. If the design does not meet the desired specifications, in block  409 , the above steps may be repeated until a sufficient shielding of the parasitic capacitance by the RF lines is achieved in block  410 . 
   It is to be understood that the present disclosure describes particular embodiments for illustrative purposes. Other embodiments are also contemplated and may include some of the following features. Optical benches may be formed from organic, ceramic or other suitable materials. Optical benches may have different sizes and shapes for the RF lines. RF choke submounts may include different orientation, size, thickness, shape and dielectric material. RF chokes may be of different size and shape. For example smaller chokes may be employed. Integrated chokes may also be employed which take advantage of the shielding effect (that is, for example, chokes that are monolithically integrated with the silicon optical bench). Designs may include RF chokes, which are directly placed on the RF line without an intermediate submount. This may mean no bonding wires are needed. Instead, the electrical contact could be established by solder or conductive adhesive, for example. 
   In other embodiments, the RF chokes may be directly placed on the optical bench without an intermediate submount. The bonding wires may be directly attached to the RF choke terminals, and the shielding may be achieved by fabricating the RF lines on a lower plane than the plane the RF choke is mounted on, e.g., a V-groove is etched in the silicon below the first terminal of the RF choke. 
   In other embodiments, the present method and system may be employed with optical or electrical outputs for a given bench setup or assembly. In other words, RF signals may be received and provide AC power for a device, while DC power may be additionally employed for yet another device, and the output may be either an electrical or optical signal. This may include the use of high frequency transistors. In addition, a whole circuit (RF and DC) may be integrated on a chip. Aspects of the present invention are not limited to optical benches and may be applied to any RF bias-T application or RF environment device. In one such example, embodiments may be applied for optical modulators or other devices. 
   Having described preferred embodiments for an impedance matching circuit with simultaneous shielding of parasitic effects for transceiver modules (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.