Abstract:
The invention relates to RF termination for reducing electrical signal reflections at the end of transmission line electrodes on an electro-optical (EO) optical modulator. The disclosed termination incorporates a RF tap which permits the monitoring of the RF power and reflection conditions at the EO modulator. The integrated termination/tap can also be integrated with detection circuitry, such as RF diodes and passive components, giving improved performance, lower cost manufacturability as well as a more compact and efficient package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 60/862,062 filed Oct. 19, 2006, which is incorporated herein by reference. 
     
     TECHNICAL FIELD 
       [0002]    The present invention relates to an integrated RF termination for an electro-optic (EO) modulator which acts both as a RF termination and RF power divider for RF signal monitoring. 
       BACKGROUND OF THE INVENTION 
       [0003]    To properly operate an electro-optic (EO) modulator, minimum RF reflection at the input port (in most cases it is the input connector) is one of the most important specifications since the RF reflection could destabilize the operation of the RF signal source if the RF reflection level is too high, or corrupt the signal fidelity by adding out of phase reflected artifacts to the signal path. There are many sources of reflection in a packaged EO modulator, however, major ones are caused by three positions, i.e. the RF termination, interfaces between input connector-to-input board and input board-to-modulator chip. 
         [0004]    The main function of the RF termination is to electrically match the impedance of the EO modulator electrode throughout the frequency band of operation to ensure minimum RF reflection at the EO modulator input port. The input connector is the packaged EO modulator&#39;s interface to the outside world. The input board is the interface between the modulator chip and input connector. The RF termination absorbs the remaining RF power with minimum perturbation to the operation of the EO modulator. 
         [0005]    If the impedance of the RF termination perfectly matches the impedance of the EO modulator electrode throughout the operating frequency band, there will be no reflection of RF power back to the RF input port. Consequently, an adequate impedance characteristic over the operation frequency band is probably the most important feature of the RF termination. To serve as a good RF termination for EO modulator applications, other important features have to be considered such as RF power handling (or dissipation) capability, temperature stability, ease of manufacturability, small size, low cost and low parasite parameters, etc. 
         [0006]    An electrical signal typically in the RF band is fed into the input connector. The input board provides an electrical transition between the input connector and EO modulator chip, where the optical wave propagating in the optical waveguide is modulated by the electrical or RF wave propagating in the electrodes through the Electro-Optical effect. The unused or remaining electrical or RF power is dumped into the RF terminal in the end of the electrode. 
         [0007]    With reference to  FIG. 1 , an example of a simplified prior art optical communication system  100  is shown, utilizing an EO modulator  107  of the present invention. The optical communication system  100  comprises a transmitter  110 , a receiver  109  and a transmission medium  108 , which connects the transmitter  110  to the receiver  109 . The transmission medium  108  is typically an optical fiber. 
         [0008]    The transmitter  110  includes a laser  104 , which operates in accordance with laser control signals received from a laser controller  103 . 
         [0009]    A lensed optical fiber, or fiber pigtail,  113  receives the optical signals  112 . The lensed optical fiber  113  is coupled to the isolator  105 , which reduces optical reflections directed towards the laser  104 . In one embodiment, the optical isolator  105  is combined with a polarizer (not shown) to further reduce reflections to the laser  104 . In another embodiment, the lensed optical fiber  113  is coupled directly to the EO modulator  107 , rather than through the isolator  105 . 
         [0010]    The EO modulator  107  receives the optical signals  112  from the laser  104  via an input fiber  106 . The EO modulator  107  includes two waveguides  114  and  115 . The controller  102  controls each waveguide  114 ,  115  independently of the other or with one control signal. The optical signals  112  are received at an input  116  of the EO modulator  107  and are modulated in each of the optical waveguides  114  and  115 . Modulated optical signals from each of the optical waveguides  114  and  115  are combined into a modulated optical signal at an output  117  of the EO modulator  107 . The EO modulator  107  may perform either amplitude modulation or phase modulation or some combination to “chirp” the light of the received optical signals  112 . The combined, modulated optical signal is transmitted across the fiber  108  to the receiver  109 . 
         [0011]    The controller  102  receives digital data signals from a data source  101  via a transmission line  118 , and generates modulation control signals in response to the received signals. The modulation control signals are introduced into the EO modulator  107  via leads  119  and  120 . The modulation control signals are indicative of a predetermined modulation of the optical signals  112  and of desired modulation chirp parameters. For example, the modulation control signals are received by the EO modulator  107 , and in response, the relative propagation velocities of each of the waveguides  114  and  115  changes to generate a desired modulation chirp parameter value. A single control signal may interact asymmetrically with waveguides  114  and  115  to produce a fixed amount of chirp. 
         [0012]    The controller  102  also introduces a bias signal via lead  121  to the EO modulator  107  which sets its operating point. The bias signal may be either preset or generated in response to changing environmental conditions such as temperature, bias drift or charge accumulation in the vicinity of the electro-optic waveguides. 
         [0013]    The prior art system described above could advantageously incorporate a RF termination according to the invention disclosed herein within the EO modulator  107 , from which an additional RF output monitoring line  122  would connect to RF detection or monitoring circuitry  123 . 
         [0014]      FIG. 2  illustrates a top planar view of a prior art packaged EO Mach-Zehnder modulator of the optical communication system  100  of  FIG. 1 . A fiber optic cable  206  is in optical communication with an optical input  216  of modulator chip  207 . The fiber optic cable  206  presents an optical signal from a light source or laser (not shown) to the input  216 . The optical signal is split into two equal signals by a first optical Y-connection  225 . RF electrodes  226  and  227  form an electrical transmission line for transmitting RF signals from an electrical input port  201  to an electrical output port  202 . The RF signals are supplied by an external source through RF interconnect board  228  connected to the electrical input port  201 . As the split optical signals propagate along optical waveguides  229  and  230 , they are modulated by the electrical field of the RF signal. The distance in which the RF signals interact with, or modulate, the split optical signals is known as the interaction distance, and is determined primarily by the modulator design. 
         [0015]    There are mainly two types of Lithium Niobate (LiNOb3)EO Mach-Zehnder (MZ) modulators used, X-cut and Z-cut types.  FIG. 2  shows only the X-cut type. 
         [0016]    A second optical Y-connection  231  combines the two split optical signals into a single, modulated optical signal. A fiber optic cable  208  which is coupled to an optical output  217  of the modulator chip  207 , presents the combined optical signal to subsequent stages (not shown) of an optical communication system. 
         [0017]    The modulator chip  207  includes a substrate  234  which in one embodiment is made of X-cut lithium niobate (LiNbO 3 ) and is approximately 1000 microns (atm) thick. In another embodiment, the modulator chip  207  is made of Z-cut LiNbO 3 . 
         [0018]    The optical waveguides  229  and  230  may be created by diffusing titanium into the substrate  234 . In one embodiment, the optical waveguides  229  and  230  are formed by creating a strip or channel (not shown) in the substrate  234 , depositing titanium in the channel, and then raising its temperature so that the titanium diffuses into the substrate  234 . The optical waveguides  229  and  230  are approximately seven microns wide and approximately three microns deep. 
         [0019]    Summarizing, the prior art RF termination board  235  is located at the electrical output port  202  of the electrodes  226  and  227  to absorb any unused or residual RF power. 
         [0020]    A RF termination board  235  according to the invention disclosed herein would also include an integrated RF monitoring output port  236  for advantageously providing RF monitor signal for use in detection, monitoring or feedback circuitry in a compact and easily manufactured manner. 
         [0021]    Commonly used RF terminations for EO modulator applications are lumped element and thick or thin-film resistors. Lumped element resistors used for EO modulators are typically surface-mounted ones, as shown in  FIG. 3 . A thick or thin-film resistor as the RF termination, as shown in  FIG. 4 , is made by hybrid circuit technology. 
         [0022]      FIGS. 3   a  and  3   b  show a top planar view and cross-section along line A-A′ respectively, of a prior art example of a RF termination board  335  corresponding to the RF termination board  235  in  FIG. 2 . The RF termination board  335  is commonly formed on a ceramic substrate  236 . Other materials with similar physical and electrical properties could also be used. A short section of RF transmission line  337 , either a coplanar waveguide (CPW) or microstrip line, extends from the termination input port  302  at the edge of the RF termination board  335  to RF termination  339 . The RF termination  339  can be either a resistor or complex circuitry of resistive and reactive passive components. A surface-mounted resistor in the form of a lumped element is shown in this example. The ground electrode  338  is connected through the vias  340  to the electrical ground plate  341 . The electrical connection between the electrical output port  202  of the modulator chip  207  of  FIG. 2  and the termination input port  302  is typically by gold wire bonding. 
         [0023]    The main shortcoming of the lumped element resistor is its need for extra soldering process and the possibility of high parasitic microwave parameters. 
         [0024]      FIGS. 4   a  and  4   b  are a top view and cross-section respectively of an alternative form of the RF termination board  235  in  FIG. 2 . This RF termination board  435  has similar components as the board shown in  FIG. 3   a  except for the RF termination  339 . The RF termination  439  is a thin- or thick-film resistor connected between RF transmission line  437  and ground electrode  438 . The RF termination can also be formed by complex circuitry of resistive and reactive thin- or thick-film components. The ground electrode  438  is connected through the vias  440  to the electrical ground plate  441 . The electrical connection between the electrical output port  202  of the modulator chip  207  of  FIG. 2  and the termination input port  402  is typically by gold wire bonding. 
         [0025]    A thick or thin-film resistor as the RF termination, such as shown in  FIG. 4 , can made by hybrid circuit technology, which is much lower cost and easier manufacturability for high-volume production, better repeatability, and smaller size compared to the lumped-element resistors. However, it is difficult to obtain a good impedance match over an appreciable electrical bandwidth. 
         [0026]    The shape of the thick- or thin-film RF termination critically affects its frequency characteristics and inherent parasitics. An RF termination is not only an RF power absorber but also a transition section between the EO modulator electrode and the electrical ground. Various geometrical shapes have desirable affects on the terminating impedance match, especially when the length and size of the termination component is big enough to impact the RF impedance at higher frequencies. A tapered shape can be a desired shape for the transition of electromagnetic wave. In the example of  FIG. 5  a taper is used in the design of electrical/RF transition between two transmission lines with quite different electrical/RF characteristics. 
         [0027]      FIG. 5  shows top view of another prior art form of the RF termination board  435  in  FIG. 4   a.  RF transmission line  537  extends from termination input port  502  to RF termination  539 , which is a thin- or thick-film resistor with distributed resistance. The RF termination  439  is tapered from a narrow end at an end of the RF transmission line  437  to a wider end at its connection with ground electrode  438 . The tapered sides of the trapezoid-shaped RF termination  439  can be described by linear, quadratic, exponential and any other gradual varying curves to ensure smooth dissipation of RF power with minimum RF reflection. 
         [0028]    Typical configurations of a front-end RF signal detection scheme for EO modulators in optical fiber communication systems include a commercially available RF power splitter/coupler located in front of the input connector to the EO modulator. In this application, the coupler usually has high coupling ratio, that is, a high percentage of RF power goes through the main channel to the EO modulator, while only a small percentage of RF power, such as 1%, is coupled to the coupling channel for monitoring the RF signal. The small portion of RF power picked up by the coupling channel goes through an RF connector on the RF splitter/coupler, and fed to the input port of RF signal detection or monitoring circuitry. Then the tapped RF signal is detected by a (single) or two (balanced) RF diodes. The detected signal can be used as input signal for a RF signal detection circuit. 
         [0029]    The main shortcomings of such an arrangement is the extra RF power loss and bulky size. Some RF power is lost due to tapping before the input signal is passed to the EO modulator. Additional coupler/splitters need more space and also make the manufacturing cost higher. 
         [0030]    A configuration that attempts to address the power loss is a back-end RF signal detection scheme for EO modulators in optical fiber communication systems. A RF power splitter/coupler is located at the output port of an EO modulator. In this application, the coupler usually also has a high coupling ratio, that is, a high percentage of RF power goes through the main channel, with only small percentage of RF power, such as 1%, being coupled to the coupling channel. When the unused RF signal from the EO modulator is fed into the input terminal of the RF splitter/coupler, the majority of RF power goes through the main channel into the bulk RF resistor termination, typically 50 ohm. The small portion of RF power picked up by the coupling channel is fed to the input port of RF signal detection or monitoring circuitry, where it can be detected by a (single) or two (balanced) RF diodes. 
         [0031]    The RF signal detection circuitry, including the RF diodes and passive components, can also be integrated into the RF termination board on a ceramic substrate using the hybrid PCB technology. 
         [0032]    In the above examples, monitoring the RF conditions near the EO modulator involves bulky and possibly expensive components such as an RF power splitter/coupler and attendant connectors, which in themselves can be sources of parasitic impedances and reflections. 
         [0033]    An object of instant application is to provide a termination with an RF tap for monitoring integrated on the same chip so that improved performance over a broader modulation frequency range can be achieved. 
       SUMMARY OF THE INVENTION 
       [0034]    Accordingly, the present invention relates to a RF termination with in-built monitoring port which can be used to optimize EO modulator or similar device performance in a compact and low parasitic configuration. 
         [0035]    Another aspect of the present invention relates to a RF termination which is based on a distributed resistance with a specific shape so that the input impedance of the RF termination and the output impedance of the monitoring port can be tailored to particular requirements. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0037]      FIG. 1  shows a block diagram of an exemplary prior art fiber-optic telecommunications system including a laser diode, an external modulator and a photodetector diode, as is well-known in the field for transmitting optical signals over optical fiber or similar optical waveguide; 
           [0038]      FIG. 2  shows a schematic of a typical prior art packaged electro-optical (EO) modulator with modulator chip, input interconnect and radio frequency (RF) termination; 
           [0039]      FIGS. 3   a  and  3   b  are prior art plan and cross-sectional views, respectively, of lumped element, typically surface-mounted, resistors used for EO modulators; 
           [0040]    Shown in  FIGS. 4   a  and  4   b  are prior art plan and cross-sectional views, respectively, of a thick or thin-film resistor as the RF termination; 
           [0041]      FIG. 5  presents a prior art plan view of a taper used in the design of electrical/RF transition between two transmission lines with quite different electrical/RF characteristics; 
           [0042]      FIG. 6  is an equivalent circuit schematic of a RF signal monitoring scheme in EO modulator applications according to the present invention; 
           [0043]      FIG. 7  is a plan view of an RF termination and RF power divider combined in one element according to the present invention; and 
           [0044]      FIG. 8  gives a plan view showing design dimensions for a trapezoidally shaped distributed resistor. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    The invention described herein combines the RF termination and RF power divider resistors in one element. The main goal is to make the integrated element into an RF termination that has excellent RF characteristics with low RF reflection and low parasitic parameters over the frequency band of operation. As an integrated termination resistor, it also acts as an RF power/voltage divider. Besides superior electrical and RF performance, the additional advantages of such a configuration are ease of design, small size, easy manufacturability, good temperature stability and low cost. 
         [0046]    An improved scheme for concurrent RF termination and RF signal monitoring is shown as an equivalent circuit in  FIG. 6 . Residual RF power from an EO modulator enters the RF chip  651  via input port  652 , from where it is conducted to RF power/voltage divider  655 . The RF power/voltage divider  655  in such an arrangement is commonly formed by two resistors  656 ,  657  with a certain ratio between them according to the desired tapping value or split ratio. The tapped residual RF power is brought out at a monitor output port  658 . A RF termination resistor  653  is connected between the input port  652  and ground connection  654 . 
         [0047]    Since the RF chip  651  is to be connected to an electrical transmission line output electrode of the EO modulator, the residual RF power from the EO modulator does not contribute additional RF power loss to the EO modulator. The resistors  656 ,  657  of the RF power/voltage divider  655  for RF power monitoring can be constructed in the form of either lumped element or thick/thin-film resistors, integrated with a RF termination resistor  653  along with RF signal detection circuitry on the same PCB. Therefore this kind of arrangement can greatly reduce the component size and manufacturing cost. 
         [0048]    However, in this scheme the RF termination resistor  653  and RF power/voltage divider  655  comprising resistors  656 ,  657  are separate elements but electrically connected in parallel, as shown in  FIG. 6 . These two elements have quite different requirements in terms of electrical/RF performance. The RF termination resistor  653  is usually designed to perform good matching to the input port  652  over the frequency band of operation, while the requirement for the RF power/voltage divider  655  comprising resistors  656 ,  657  has its own requirements, such as stable pickoff ratio, ease of trimming or setting the resistor values and providing adequate source impedance at the monitor output port  658  to the monitoring diodes or other electronics. It is very difficult to design a broadband performance RF termination in circuit parallel with a good quality RF power/voltage divider. 
         [0049]      FIG. 7  shows the top planar view of an integrated RF termination board  735 . Residual RF power coming from an EO modulator (not shown in the figure) is applied to input port  702  which is connected to a RF transmission line  737 . Typically multiple wire bonding is used to connect an EO modulator electrical output electrode to the RF transmission line or CPW  737 , as well as an EO modulator electrical ground electrode to termination ground plate  748 . The termination ground plate  748  connects to a termination electrical ground through vias  750 . 
         [0050]    The structure of the integrated RF resistor  739  is shaped to gradually taper out in a widening taper and reverse to gradually taper back in with a narrowing taper. Since the propagating resistance of a longitudinal slice of resistive material is inversely proportional to the width of the structure, the gradually increasing or decreasing width provides an ever decreasing resistance as the characteristic impedance of the taper gradually increases. The tapered structure provides a traveling wave RF impedance change, much in the same way that impedance tapers are used to match one RF impedance to another. An example would be converting from a 50 ohm characteristic impedance coaxial cable to a 75 ohm coaxial cable through use of a tapered transmission line. As a result, power is slowly dissipated down the length of the structure, as its impedance slowly decreases. In the case of the structure that tapers back down, the impedance is increased along its length, still continuing to dissipate RF power. The taper out—taper in structure provides the combination of lossless impedance contribution along with lossy impedance contribution, that produces RF power dissipation, and optimized RF impedance match. 
         [0051]    The RF resistor  739  approximating a kite shape with truncated vertices can be considered as an electrical transmission line with varying width as it extends from the RF transmission line  737  to the a second ground plate  738 . The width increases monotonically to an intermediate stage  760 , where its width is constant, thereafter decreasing monotonically to where it connects to the second ground plate  738 , which in turn is connected to the termination electrical ground through vias  740 . 
         [0052]    An RF tap output port is provided by RF tap electrodes  762 , preferably in the form of metal bars, are connected to both sides of the RF resistor  739  at the intermediate stage  760 . The tapered edges  761 ,  763  of the RF resistor  739  can be tailored to achieve desired characteristics such as smooth dissipation of RF power with minimum RF reflection. The profile of the tapered edges  761 ,  763  can be described by linear, quadratic, exponential and any other gradually varying functions of distance along the transmission line. 
         [0053]    The RF resistor  739  can be formed from carbon-filled polymer with certain sheet resistance value. The precise target resistance value of the resistor can be realized by laser trimming. 
         [0054]    The RF power dividing ratio or split ratio can be realized by adjusting the size of upper and lower trapezoidal portions of RF resistor  739 . The tapped RF power from the RF tap electrodes  762  is fed to the input of the RF signal detection circuitry (not shown in the figure). The RF signal detection circuitry, including the RF diodes and passive components, can also be integrated into the RF termination board  735  on a ceramic substrate  734  using the hybrid PCB technology. 
         [0055]    Values for the split ratio can generally lie in the range of −6 dB to −20 dB, but about −10 dB is preferred for most practical applications. 
         [0056]    For the input port  702  the value of input impedance is designed to provide optimal RF matching with transmission line electrode impedance on an electro-optical (EO) optical modulator. While the impedance values vary with a particular EO modulator design, commonly ranging from about 30 ohm to about 75 ohm, about 40 ohm is typical on most LiNbO 3  EO modulators. The RF tap electrodes  762  on the other hand generally operate into standard 50 ohm RF circuits, so their source impedance is designed to match this value. 
         [0057]    The DC/low frequency resistance of RF resistor  839 , shaped approximately as an asymmetric rhombus with possibly curved sides, as shown in  FIG. 8 , can be estimated using the integral equation or summation of small slices in the horizontal direction by equation: 
         [0000]        R   i   =ΣR   s *δ h   /[t*f ( x   i )], 
         [0058]    where R s  is the sheet resistance of the resistive material, δh is the height of the slice, t is the thickness of the resistive material and f (x i ) is the function of the curves for the i th  slice. 
         [0059]    An input end  827  forms the top, while a grounding end  828  forms the bottom and tap ends  829  are on both sides of the structure of the RF resistor  829 . The tapered edges  860 ,  870  may be straight, or curved inwards as shown by the dashed lines  861 ,  871  according to the function f(x i ). 
         [0060]    Consider the RF resistor  839  as consisting of three portions: two approximately trapezoidal structures and one central rectangular structure, as shown in  FIG. 8 . Using the equation above, the DC/low frequency resistance R 1  and R 2  can be calculated for the upper and lower portions, respectively. Similarly, the resistance of the rectangular portion, R r , can also be calculated. Thus the total DC/low frequency resistance is a summation of R 1 , R 2  and R r . 
         [0061]    The voltage dividing ratio required for proper operation of a RF signal detection circuit is R 1 /(R 1 +R 2 ). In the structure, the total height, H, is the summation of the heights of the two trapezoidal portions, h 1  and h 2 , and the rectangular sheet, h 3 . Other parameters of the RF termination are width, W, and sheet resistivity, R s . 
         [0062]    With adjustment of the width, W, the heights of three sections, h 1 , h 2  and h 3 , and the sheet resistivity, R s , the total resistance value for the desired RF termination and the voltage/power dividing ratio for RF signal detection can be individually optimized. 
         [0063]    An integrated RF termination has been described for EO modulator applications with the functionality of RF termination and RF power/voltage divider for RF signal detection. The values of termination resistance and power divider ratio can be adjusted separately and optimized simultaneously. The single element component exhibits excellent RF/electrical characteristics, i.e. broad operational frequency bandwidth and low parasitic parameters. The integrated RF termination with double taper or similar shape allows a smooth transition from the impedance of the EO modulator electrode to the electrical ground with minimum reflection. 
         [0064]    The integrated RF termination disclosed herein can also be used as a stand-alone component or in other types of integrated optical modulators, such as Electro-Absorption (EA) and EO MZ modulators on semiconductor material as well as in any electro-optical or optoelectronic device, which requires either RF termination only or a combination of RF termination and RF power/voltage divider. 
         [0065]    In summary, a RF termination is disclosed for an optical modulator comprising a substrate chip having an input port for receiving a residual RF modulation signal from the optical modulator; a ground connection on the substrate chip for providing an electrical DC or RF return path; a resistive transmission line on the substrate chip having a width and a length extending from an input end to a grounded end for absorbing the residual RF modulation signal, wherein the input end is connected to the input port and the grounded end is connected to the ground connection; and a RF tap output port on the substrate chip, connected to the resistive transmission line at an intermediate stage, for coupling out a RF monitoring signal at a split ratio of the residual RF modulation signal. The split ratio is typically in the range of about −6 dB to −20 dB. 
         [0066]    The width of the resistive transmission line may have a widening taper from the grounded end to the intermediate stage and narrowing taper from the intermediate stage to the RF input port. 
         [0067]    The widening taper or the narrowing taper may follow a profile function which is one of linear, quadratic and exponential. The dimensions of the widening taper or the narrowing taper can be adjusted by laser trimming. 
         [0068]    The resistive transmission line can be made of either thin film or thick film resistive material. Carbon-filled polymer is a suitable material for the resistive transmission line. 
         [0069]    The resistive transmission line can be either a coplanar waveguide or a microstrip line. 
         [0070]    The RF tap output port has an impedance in the range of about 30 ohm to 75 ohm. 
         [0071]    The substrate chip comprises one of ceramic and semiconductor. Hybrid PCB technology can be used to fabricate the substrate chip.