Abstract:
An optoelectronic system includes an optical signal modulator, an optical input guide and an optical output guide connected to the optical signal modulator. The system further includes a reflective optical element in the optical signal modulator, the element disposed to reflect an input light beam incident through the optical input guide into an output light beam through the optical output guide. The system further includes electrical terminals in the optical signal modulator. The electrical terminals are configured such that an electrical signal on the electrical terminals is operable to interact with the input light beam.

Description:
TECHNICAL FIELD 
   The invention relates to remote electronic testing and measurement and more particularly to systems and methods for remotely stimulating and measuring electronic signals through a fiber optic cable. 
   BACKGROUND OF THE INVENTION 
   When electrical signals being sourced or sensed are rapidly varying, i.e. when they contain high frequency components, it is often difficult to deliver these signals to or from a remote location using conventional means. Typically, high frequency signals are delivered via coaxial cable or other electrical transmission line, with or without the aid of repeater amplifiers. Conventional transmission lines are frequency dispersive, meaning that they distort the delivered signal relative to the sourced or sensed signal by attenuating or excessively delaying higher frequency components of the signal relative to the lower frequency components. Also, conventional transmission lines can distort a signal by means of reflections caused by impedance discontinuities along the line. A certain degree of distortion can be tolerated in a communication system, whereas in a measurement system signal distortion must be minimized. Further, in cases where a large dc voltage difference exists between the test instrument and the device being tested or in cases where the presence of a conducting element such as a coaxial cable can disturb the measurement, as for example antenna testing, connections such as coaxial cable are impractical. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the invention, an optoelectronic system is provided. The system includes an optical signal modulator, an optical input guide and an optical output guide connected to the optical signal modulator. The system further includes a reflective optical element in the optical signal modulator, the element disposed to reflect an input light beam incident through the optical input guide into an output light beam through the optical output guide. The system further includes electrical terminals in the optical signal modulator. The electrical terminals are configured such that an electrical signal on the electrical terminals is operable to interact with the input light beam. 
   In accordance with another embodiment of the invention, a method of remote delivery of a modulated signal is provided. The method includes modulating an input light beam with an electrical signal using optical signal modulation, and reflecting the modulated light beam into an output light beam direction different from that of the input light beam. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a system for delivering a copy of an electrical response signal from a remote electrical device under test to an electronic measurement instrument by means of a fiber optic cable, in accordance with embodiments of the invention; 
       FIG. 2  depicts a system for remote delivery over an optical fiber link of a modulated signal for stimulating a remote electrical device, in accordance with embodiments of the invention; 
       FIG. 3  depicts a system for concurrent delivery over an optical fiber link of stimulus and response signals both to and from a remote electrical device, in accordance with embodiments of the invention; 
       FIG. 4A  depicts a modulator configuration, including an electroabsorption modulator (EAM), which controls the transmission intensity of a light beam in proportion to applied control voltage; 
       FIG. 4B  depicts a reflection-mode EAM (REAM) in accordance with embodiments of the invention, combining an EAM with a reflecting surface; 
       FIG. 5  is a graph of a transfer curve of reflected power-vs-voltage of a typical REAM for constant incident power, in accordance with embodiments of the invention; 
       FIG. 6  depicts a quantum well modulator structure, one example of electroabsorption modulator (EAM); 
       FIG. 7A  depicts a REAM configuration biased for linear operation, with AC coupled input signal, in accordance with embodiments of the invention; 
       FIG. 7B  depicts a differential pair REAM configuration, in accordance with embodiments of the invention; and 
       FIGS. 8A-8E  illustrate REAM configurations for a variety of applications. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   It is often necessary or desirable to sense an electrical signal at a location remote from the measurement instrument that processes the sensed data. Likewise, it is often necessary to deliver an electronic signal to a location remote from the source of the signal. Indeed, it is sometimes desirable to both sense and deliver signals remotely with the same physical apparatus, so that devices being tested can be fully characterized in an efficient manner. 
   Over distances conventionally used for remote testing (meters to hundreds of meters), single-mode fiber optic cable is typically of low enough dispersion to cause negligible signal distortion. Also, photodetectors and reflection-mode electroabsorption modulators (REAMs) can be designed to have low signal distortion over useful ranges of signal level. Thus, the distance between electronic measurement instrument and location of remote device under test can be increased to distances much greater than possible with conventional electrical transmission lines, for example coaxial cables. 
     FIG. 1  depicts system  100  for delivering a copy of an electrical response signal from a remote electrical device under test to an electronic measurement instrument by means of a fiber optic cable, in accordance with embodiments of the invention. In the remote sensing configuration shown in  FIG. 1 , continuous-wave (CW) light beam  11  launched by CW light source  101  passes through optical circulator  102  and through optical guide  104  as CW light beam  12 . CW light beam  12  is modulated in an optical signal modulator, for example Reflection Mode Electroabsorption modulator (REAM)  105  at the remote sensing point by remote electrical signal  13  generated by remote electrical device  107 . Bias voltage  106  is typically applied to REAM  105  to provide linear operation. Modulated light beam  14  is then reflected by REAM  105  back through optical guide  104 , optical circulator  102 , and optical guide  108  as modulated light beam  15 , which is then typically amplified by optical amplifier  109  to provide amplified light beam  16  through optical guide  111 . This amplified modulated light beam  16  is typically detected by photodetector  110 , which generates photoelectric signal  17  through electrical cable  112 . Photoelectric signal  17 , which replicates remote electrical signal  13 , is typically processed by electronic measurement instrument  113 . 
     FIG. 2  depicts system  200  for remote delivery over an optical fiber link of a modulated signal for stimulating a remote electrical device, in accordance with embodiments of the invention. Modulated light signal  21  from modulated light source  201  passes through optical fiber link  103 , optical circulator  102 , and as modulated light signal  22  through optical fiber link  104 , and impinges on Reflection Mode Electroabsorption Modulator (REAM)  205 . REAM  205  can be the same as REAM  105  depicted in  FIG. 1 . REAM  205 , typically biased by negative voltage  206 , converts modulated light signal  22  to electrical signal  23 , which stimulates electrical device being tested  207 . In this case, the response of remote electrical device  207  can be monitored by other means, such that circulator  102 , optical amplifier  109 , photodetector  110 , and electronic measurement instrument  113  need not be used. These elements are shown to illustrate the fact that the same apparatus can be used either for stimulus testing as in  FIG. 2  or for response testing as in  FIG. 1 , with the difference being whether the light source is CW or modulated. 
     FIG. 3  depicts system  300  for concurrent delivery over an optical fiber link of stimulus and response signals both to and from a remote electrical device, in accordance with embodiments of the invention. Modulated light signal  21  from modulated light source  201  passes through optical fiber link  103 , optical circulator  102 , and as modulated light signal  22  through optical fiber link  104 , and impinges on Reflection Mode Electroabsorption Modulator (REAM)  305 . REAM  305  can be the same as REAM  205  depicted in  FIG. 2 . REAM  305 , typically biased by bias voltage  306  for linear operation, converts modulated light signal  22  to electrical signal  33 , which stimulates electrical device being tested  307 . Electrical device  307  response  34  to stimulation  33 , thereby modifying the voltage at its terminals, in turn modulates the reflection coefficient of REAM  305 . The reflection coefficient of REAM  305  interacts with incident light  22 , reflecting a modulated light replica  35  of response voltage  34  through optical guide  104 , optical circulator  102 , and optical guide segment  108  as modulated light replica  36 , which is then typically amplified by optical amplifier  109  and transmitted as amplified modulated light replica  16  through optical guide segment  111  to photodetector  110  and processed by electronic measurement instrument  113 . A typical application of the configuration shown in  FIG. 3  is Time Domain Reflectometry, but other combined stimulus/response applications could also be performed, for example Network Analysis (Frequency-domain testing of electronic networks). 
   The acronym REAM represents “Reflection-mode Electroabsorption Modulator.” A REAM is essentially an electroabsorption modulator configured to operate in reflection mode, for example by impinging light onto a first face of the modulator and terminating the opposite face of the modulator with a mirror. Reflection modulators have been used with free space optical beams to form communication systems. 
   The optical signal modulator can be any type of reflection mode modulator, including: electrooptic (EOM); electromechanical (EMM); and electroabsorption (EAM). Use of electro-optic modulators (EOMs) to convert electrical signals to modulated light, and photodetectors to convert the modulated light to an electrical signal is standard practice in many optical communication systems. The electroabsorption modulator (EAM), which has been advantageously reduced to practice, is believed to be unique among modulator types in having the ability to function as both a modulator and a detector of light. The applications depicted in  FIGS. 2 and 3 , for example, apply only to the electroabsorption modulator type. 
     FIG. 4A  depicts modulator configuration  400  including electroabsorption modulator (EAM)  401 , which controls the transmission intensity of a light beam in proportion to applied control voltage  402 . EAM  401  modulates the transmitted light so that modulated light  42  exiting the right facet is a time-varying fraction of light  41  entering the left facet, with the time varying fraction being controlled by time varying control voltage  402 .  FIG. 4B  depicts reflection-mode EAM (REAM)  410  in accordance with embodiments of the invention, combining EAM  401  with reflecting surface (e.g., a mirror)  403 , having entrance beam  41  and modulated exit beam  43  at the left face of the modulator. 
   An advantage of the REAM geometry in sensing and probing applications is that only a single optical path is required. If, for example, the optical path is an optical fiber, then the complete assembly can be made much simpler and less complex. 
     FIG. 5  is a graph of a transfer curve of reflected power-vs-voltage of a typical REAM for constant incident power, in accordance with embodiments of the invention. Reflected power is displayed along vertical axis  502  as a function of control voltage along the horizontal axis  501 . Bias voltage is a selected value of control voltage. To bias the device near the center of the linear operating range, it is necessary to apply a negative dc bias value, for example voltage  505 . This produces an equilibrium reflected power value  504 . Linear operation is then achieved centered around bias point  503 . Devices can be built with Vbias=0, but such devices would typically have slow response of reflection to changes in modulating voltage. 
   The REAM absorbs light to varying degrees, depending on wavelength and the particular value of control voltage Vcontrol. Typically, if Vcontrol is zero volts, the REAM is said to be “transparent”, and all light is reflected. If Vcontrol is set to Vbias, then the REAM operates in a linear region, and the amount of light reflected is linearly proportional to the applied voltage. Vbias can be used for detection mode, even though detection efficiency is not maximized. If the REAM is biased, for example, Vbias=−5V, then nearly all the incoming light is absorbed. This Vbias value can be used for detection mode operation when maximum detection efficiency and bandwidth are required. 
   When the REAM is biased to Vcontrol=a negative voltage, light is absorbed, and the energy of the incoming light is used to create “hole-electron pairs” of mobile charge. These mobile holes and electrons are swept out of the electrical terminals of the REAM as a current of flowing charge (photocurrent). This photocurrent has a fast response time, so that if the incoming light is modulated in intensity, the photocurrent generated by absorbing this light is modulated also, thereby producing a useful electrical replica of the modulated light intensity. This operating mechanism, called the photoelectric effect, which is used traditionally in photodetectors, enables the REAM to be used as a photodetector. 
   A basic description of the operation of one example of electroabsorption modulator is available on the internet at:
     http://www.bell-labs.com/project/oevlsi/tutorial/   

   A particular electroabsorption mechanism seen only in quantum wells is called the “quantum-confined Stark effect”. This mechanism, like other electroabsorption mechanisms in semiconductors, is also very fast. There are no intrinsic speed limitations on the mechanism itself until timescales well below a picosecond. In practice, speed is limited only by the time taken to apply voltage to the quantum wells, which is typically limited by resistor-capacitance limits of the external circuit. Speeds of 40 GHz have been demonstrated. 
     FIG. 6  depicts quantum well modulator structure  600 , one example of electroabsorption modulator (EAM). Quantum wells  601  are undoped intrinsic semiconductor layers, sandwiched between p-doped top contact  602  and n-doped bottom contact  603  on n-doped substrate  604 . This forms a diode structure, which can be reverse biased through contacts  605 ,  606 , to apply a modulated electric field perpendicular to the quantum well layers. The electric field modulates input light beam  61  to produce output modulated light beam  62 . Structure  600  is made using gallium arsenide and aluminum gallium arsenide, working best at wavelengths of about 850 nm, although other semiconductor materials may be used. Quantum well structure and operation are further described, for example, in D. A. B. Miller, “Optoelectronic applications of quantum wells,” Optics and Photonics News 1, No. 2, pp 7-15, February 1990. 
   Electrooptic modulators are typically based on the quadratic (Kerr) electrooptic effect, which is exhibited by crystals such as potassium tantalate niobate or barium titanate. Other electrooptic modulators are based on the linear (Pockels) electrooptic effect, which occurs in crystals such as potassium dideuterium phosphate or cesium dideuterium arsenate. Electromechanical modulators are based on the principles of stress birefringence or photoelasticity, and include a class of acoustooptic modulators using material such as crystalline quartz. Electrooptic and electromechanical modulation are summarized in Hecht, “Optics,” Second Edition, Addison-Wesley Publishing Company, pp. 314-321, 1987. 
     FIG. 7A  depicts REAM configuration  700  biased for linear operation, with AC coupled input signal, in accordance with embodiments of the invention. Basic REAM device  701  is diagrammed as an electronic diode. Bias voltage  71  is applied through bias resistor  703 , and electrical signal  72  is coupled in/out through AC coupling capacitor  702 . Reference voltage  73  is supplied to REAM  701 . Light beams  74 ,  75  modulate/are modulated by electrical signal  72 . As with many electronic devices, the REAM is typically biased, as described in connection with  FIG. 5 , and the proper bias voltage does not necessarily coincide with the voltage being sensed. In many cases, AC coupling between the REAM and the voltage under test is required. 
     FIG. 7B  depicts differential pair REAM configuration  710 , in accordance with embodiments of the invention. One way to configure a DC-coupled REAM sensor is to arrange basic REAM devices  701  to operate in differential pairs. Negative bias voltage  71  is applied through respective bias resistors  703 ,  704 , positive bias voltage  70  is applied through bias resistor  705 , and electrical signals are connected at  72 ,  73 . Signals can be either modulated voltage signals or DC reference voltages. Light beams  74 - 77  modulate/are modulated by electrical signals  72 ,  73 . 
     FIGS. 8A-8E  illustrate REAM configurations for a variety of applications.  FIG. 8A  depicts REAM configuration  800 , in which voltage from signal  807  and ground  805  is applied to REAM  801  through contacting probe tips  804 ,  806  in housing  808 , in accordance with embodiments of the invention. Input and exit light beams  81 ,  82  are coupled through lens  802  between optical fiber  803  and REAM  801 . REAM configuration  800  has high input impedance, such that it presents minimal electrical loading to the signal line. 
     FIG. 8B  depicts REAM configuration  820 , in which voltage  810 ,  812  is applied to REAM  801  through non-contacting, electrostatic-coupled probe tips  809 ,  811  in housing  808 , in accordance with embodiments of the invention. Input and exit light beams  81 ,  82  are coupled through lens  802  between optical fiber  803  and REAM  801 . REAM configuration  820  has high input impedance, such that it presents minimal electrical loading to the signal line. 
     FIG. 8C  depicts REAM configuration  840 , in which voltage is applied to REAM  801  through impedance matching network  813  attached to coaxial connector  814 , in accordance with embodiments of the invention. Input and exit light beams  81 ,  82  are coupled through lens  802  between optical fiber  803  and REAM  801 . REAM configuration  840  presents a matched load to the coaxial connector (typically 50 ohms). 
     FIG. 8D  depicts REAM configuration  860 , in which voltage is applied to REAM  801  through terminals of antenna  815 , in accordance with embodiments of the invention. Antenna  815 , as traditionally configured, may be a single antenna having at least two terminals, one of which can be ground. Matching network  813  transforms the REAM impedance to an optimal load for antenna  815 . Input and exit light beams  81 ,  82  are coupled through lens  802  between optical fiber  803  and REAM  801 . 
     FIG. 8E  depicts REAM configuration  880 , in which voltage is applied to REAMs  801  through electromagnetic wave directional coupler  816 , in accordance with embodiments of the invention. The voltage is proportional to the traveling wave amplitude on electromagnetic line  817 . Input and exit light beams  81 ,  82  and  83 ,  84  are coupled through lenses  802  between optical fibers  803  and REAMs  801 . 
   While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.