Abstract:
An input sampler interface to a track and hold circuit that decouples a high bandwidth (possibly optical domain) input signal from a lower bandwidth electrical domain of a subsequent track and hold circuit or other circuit.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. H98230-05-C-0472. The Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     This application is directed to decoupling the bandwidth of an input pulse, either electrical or optical, from the bandwidth of a subsequent circuit. The subsequent circuit may be a track and hold circuit that is itself the front end of an analog to digital converter. 
     BACKGROUND 
     This invention describes a method and apparatus for capturing the information in a short pulse. Generally the pulse is electrical or an optical pulse passed through a photo detector. Generally the pulse is the input to a lower bandwidth, electrical domain, analog to digital converter. 
     Ordinarily, an optical signal is directed to a photo detector and the photo detector output is sent to a track and hold circuit. The track and hold circuit output is the input to an analog to digital converter. After each conversion of the track and hold output, a new sample of the input is captured by the track and hold. In terms of bandwidth, a short input optical pulse requires the track and hold circuit to have a bandwidth inversely proportional to the duration of the input pulse. For a 5 picosecond input pulse, the bandwidth of the track and hold is approximately 200 GHz. Such a broad bandwidth presents its own design issues. In particular a broad bandwidth incurs a substantial penalty in signal to noise ratio. 
     A continuous time delta-sigma analog to digital converter can efficiently capture the information from a photonic sampler, but such Analog to Digital Converters (ADC) have limited bandwidth. A simple integrating sampler can capture the information from a photonic sampler but is a poor match to the quantizer portion of an ADC because the fraction of the sample period during which the integrator can drive the quantizer is limited by the time required to reset the integrator between samples. Alternatively, a peak detector in place of an integrator could be used but its bandwidth would be on the order of the input pulse duration with the commensurate noise such a bandwidth implies. 
     There is a need for an interface circuit that will capture the information in the input pulse and preserve it for the track and hold circuit, without the track and hold circuit having a bandwidth dictated by the duration of the pulse. 
     SUMMARY 
     The problem of the track and hold circuit having enough bandwidth, such that it can capture a fast electrical pulse or the output of a photo detector driven by an optical pulse, is overcome by using a sampler, controlled independently of the track and hold circuit, to decouple the bandwidth of the track and hold circuit from the bandwidth of the input pulse. In some embodiments the sampler is an integrator. In other embodiments the sampler may be a peak detector. 
     In a first embodiment, a photonic analog to digital converter sampler apparatus comprising: a sampler; wherein the sampler is enabled by an sampler enable signal and reset by an sampler reset signal; an output of the sampler is connected to an input of a subsequent circuit; wherein the subsequent circuit is controlled by a subsequent circuit enable signal. The sampler enable, sampler reset and subsequent circuit enable signals are synchronized to decouple the bandwidth of the subsequent circuit from the bandwidth of an input pulse to the sampler. 
     In a second embodiment, the invention of the first embodiment further comprising an amplifier amplifying the input to the sampler. 
     In a third embodiment, the invention of the first embodiment where the sampler is an integrator. 
     In another embodiment, the invention of the third embodiment further comprising a photo detector where the photo detector converts an optical pulse to an electrical pulse for either the sampler or an amplifier. 
     In another embodiment, the invention of the first embodiment where the subsequent circuit is a track and hold circuit. 
     In another embodiment, a method for decoupling the bandwidth of a subsequent circuit from an optical input comprising: converting the optical input pulse into an electrical signal; sampling the electrical signal wherein the sampler is controlled by a sampler enable signal and reset by a sampler reset signal; capturing the output of the sampler in response to a subsequent circuit enable signal. The subsequent circuit may be a track and hold circuit. 
     In another embodiment, the method of the previous embodiment wherein the sampler is an integrator. 
     In another embodiment, the method further comprising amplifying the electrical signal and passing the amplified electrical signal to the sampler. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with reference to the following drawings where: 
         FIG. 1  shows a block diagram of the Photonic Analog to Digital Input Sampler 
         FIG. 2  illustrates the timing of the Photonic Analog to Digital Input Sampler. 
         FIG. 3  illustrates an embodiment of the Photonic Analog to Digital Input Sampler 
     
    
    
     DESCRIPTION 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and general principles defined herein may be applied to a wide range of embodiments. Thus the invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without necessarily being limited to specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
     All features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalents or similar features. 
     The drawings and accompanying descriptions are meant to provide the structure for the function performed by the components described in the drawings and accompanying descriptions. 
     Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 USC Section 112, Paragraph 6. In particular, the use of step of or act of in the claims herein is not intended to invoke the provisions of 35 USC Section 112 Paragraph 6. 
     One alternative to a broad bandwidth track and hold circuit is to use a sampler between the optical signal and the track and hold circuit. By controlling the sampler, such that it is switched on when an optical pulse is expected but not reset until the track and hold circuit has finished with its task, the bandwidth of the track and hold circuit can be substantially decoupled from the optical pulse duration. This is because the sampler alone needs time to accomplish its reset operation. The preferred embodiment uses an integrator as the input sampler to integrate the input optical pulse and preserve it for the track and hold circuit. A person skilled in the art will realize that other circuits than an integrator may be used as a sampler as long as the alternative circuit may be enabled by one signal and reset by another. 
     As shown in  FIG. 1 , the optical signal  10  typically drives a Photo Detector circuit  110 . The Photo Detector  110  circuit may be implemented with photo diodes, photo transistors, photo resistors or similar devices. However implemented, the Photo Detector has its own capacitance and resistance which acts as a low pass filter on the electrical equivalent of the optical pulse  10 . The output  20  of the Photo Detector  110  circuit may pass through an optional Amplifier  120  before driving the Sampler  130 . The Amplifier  120  may supply the gain to drive the Sampler and improve the signal to noise ratio. The Sampler circuit  130  has two controls, an Enable  30  and an Reset  40  generated by an Enable Circuit  150  and an Reset Circuit  160  respectively. The Enable  30  may cause the Sampler  130  to integrate the Amplifier output  25 . The Reset  40  will cause the Sampler  130  to reset to its initial value upon receipt. The output  60  of the Sampler  130  is captured by a Track and Hold circuit  140 . The Track and Hold circuit  140  is controlled by the Track and Hold Enable circuit  170  that generates the Track and Hold Enable signal  50 . Upon receipt of the Track and Hold Enable signal  50  the Track and Hold Circuit  140  will capture a new value and present that value on its output  70  to the subsequent analog to digital converter (not shown). 
     Various implementations of the Track and Hold circuit  140  are possible and most will be compatible with the present design as long as the Track and Hold circuit  140  is controlled by a Track and Hold Enable signal  50  and the bandwidth of the Track and Hold circuit  140  is at least approximately equal to the reciprocal of the Track and Hold Enable signal  50  period. 
     The timing interrelationships between the components and signals of  FIG. 1 , is shown in  FIG. 2 . The input optical pulse  10  duration, without implying a limitation, is on the order of 5 picoseconds. The output  20  of the Photo Detector  110  shows the low pass filtering effect of the photo detector&#39;s capacitance and resistance. The Enable  30  is synchronized with the Photo Detector output  20  such that the Sampler  130  may integrate the output  20  of the Photo Detector or if an Amplifier  120  is used, the output  25  of the Amplifier  120 . Alternatively the Sampler  130  may capture the peak of the output  25  of the Amplifier  120 . 
     In an alternative embodiment, as shown in  FIG. 1 , the Sampler  130  captures an electrical pulse  12  instead of an optical pulse  10 . 
     The Sampler  130  output  60  is shown in  FIG. 2 . The Sampler  130  output  60  persists until the Reset signal  40  is received. The Track and Hold Enable signal  50  is synchronized with the Reset  40  and Enable  30  signals but offset in phase such that the Track and Hold circuit  140  can capture the Sampler  130  output  60 . Every occurrence of the Track and Hold Enable signal  50  causes the Track and Hold circuit  140  to capture a new value of the Sampler  130  output  60  as Track and Hold output  70  for the subsequent analog to digital converter (not shown). Since the Sampler  130  presents a signal  60  to the Track and Hold  140  that persists until the Reset signal  40  arrives and is not dependent on the duration of the Optical Pulse  10  or the Electrical Pulse  12 , the Track and Hold  140  circuit does not need a bandwidth commensurate with the Optical Pulse  10  or the Electrical Pulse  12 , but rather commensurate with a bandwidth with the Track and Hold Enable signal  50 . In effect, the bandwidth of the Track &amp; Hold circuit  140  is decoupled from the bandwidth of the input Optical Pulse  10  or the input Electrical Pulse  12 . 
     That the bandwidth of the Track and Hold circuit  140  is decoupled from the input Optical Pulse  10  is seen by comparing the period of the Track and Hold Enable signal  50  with the duration of the Optical Pulse  10 . The values shown in  FIG. 2  are illustrative and not intended to be limiting. The bandwidth of the Track and Hold circuit  140  is approximately 1/260 ps or 3.8 GHz. The input Optical Pulse has a bandwidth of approximately 200 GHz. 
       FIG. 3  shows an embodiment of the Sampler circuit invention as an integrator implemented with bipolar transistors and capacitors CO and Cl. One skilled in the art will realize different technologies will allow for functionally comparable circuits of greater or less complexity. For example, the integrator reset circuitry comprising bipolar transistors Q 7  through Q 17  may be replaced by one or more field effect transistors. The implementation shown in  FIG. 3  is not meant to imply any limitation to a particular technology.