Abstract:
A wideband communications system uses a photomixer, a radio frequency (RF) mixer, and an antenna. The photomixer is comprised of high speed phototransistors that are illuminated by two laser beams. The laser beams are generated by two lasers, one tunable, and conveyed to the photomixer via fiber optic cables. The two beams are mixed by the photomixer to generate a radio heterodyne signal that is mixed with antenna signals to generate an intermediate frequency (IF) signal based on the differences of the generated heterodyne frequency and the antenna signals. Conventional fiber optic means may be used to convey the IF signals, along with the fiber optic cables to the photomixer, to and from a remote RF head located at some distance from the rest of the communications system to overcome losses in transmission lines.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to communications systems, microwave communications systems, and specifically an electro optic microwave communications system. 
   As the information age has grown, the availability of wireless bandwidth has been seriously constrained by available spectrum. The obvious growth path, as spectrum becomes fully occupied or oversubscribed, is to move upward in frequency to unused bandwidth. The upper frequency limit has steadily risen to where the use of the millimeter and terahertz (THz) spectrum is now realistic. Building radio frequency (RF) circuitry for these frequency ranges is very difficult and expensive and in particular a unique system must be built for each sub-band. No known technology exists for generation (transmit) and reception (receive) of signals across multiple sub-bands or indeed across the entire band (up to 1 THz). Various absorption holes exist in the THz frequency range and hence useful communications is possible in these holes. Government surveillance systems are already vitally concerned with signal activity in this range and are hampered by the lack of RF technology to prosecute such signals. 
   The state-of-the-art today finds commercial communications up to 67 gigahertz (GHz), and military uses to 94 GHz and beyond. Much experimentation is underway in the terahertz range, and various specialized users are already demanding receive capability in the 100- to 300-GHz range. From a pure atmospheric attenuation perspective shown in  FIG. 1 , it can be seen that there are several low-attenuation bands at 140, 240 and 560 GHz usable for free space communications. Many uses, however, do not rely on low-attenuation bands, and can utilize any terahertz frequency for deliberate limited range communication. 
   Current technology is slowly evolving toward an expanded set of block converters, each using a unique technology optimized to a particular frequency band to cover the terahertz frequency range. Typically, the block converter includes an antenna, preamplifier and mixer driven by a fixed, multiplied local oscillator. Two or more block converters may be used in cascade. 
   What is lacking is the technology to implement a continuous-tuning communications or surveillance system that covers the 20–500 GHz band, similar to the common availability today of receivers that cover 20–520 megahertz (MHz). 
   SUMMARY OF THE INVENTION 
   An electro optic microwave communications system receiver for receiving a high frequency microwave transmitted signal is disclosed. The receiver comprises an antenna to receive the transmitted signal and to provide a received signal. A photomixer is connected to the antenna to provide an injection signal to mix with the received signal to down convert the received signal to an intermediate frequency (IF) signal. A fixed laser illuminates the photomixer at a first frequency and a tunable laser illuminates the photomixer at a second frequency. At least one fiberoptic cable connects the fixed laser and the tunable laser to the photomixer. The photomixer mixes the fixed laser and the tunable laser to provide the injection signal at a frequency equal to the difference of the first frequency and the second frequency. 
   In a first embodiment of the electro optic microwave communications system receiver a mixer connected between the antenna and the photomixer mixes the received signal with the injection signal to down convert the received signal to an IF signal. This embodiment of the electro optic microwave communications system receiver may comprise a fast Fournier transform (FFT) connected to the photomixer to eliminate image responses and provide an output. 
   In a second embodiment of the electro optic microwave communications system receiver the photomixer mixes the injection signal with the received signal to down convert to an IF signal. This embodiment of the electro optic microwave communications system receiver may further comprise an image reject filter connected to the antenna to reject receiver image response signals and a low amplifier connected between the image reject filter and the photomixer to amplify the received signals. 
   In the electro optic microwave communications system receiver a first fiberoptic cable may be used to connect the fixed laser to the photomixer and a second fiberoptic cable may be used to connect the tunable laser to the photomixer. 
   In the electro optic microwave communications system receiver an optical combiner may be connected to the fixed laser and the tunable laser and one fiber optic cable is connected to the optical combiner and the photomixer. 
   The electro optic microwave communications system receiver further comprises a laser IF modulator connected to the photomixer to modulate a laser beam with the IF signal. An IF fiberoptic cable is connected to the laser IF modulator to transmit the IF modulated laser beam to a laser IF demodulator that demodulates the IF modulated laser beam. 
   In the electro optic microwave communications system receiver the length of the one fiberoptic cable and the IF fiberoptic cable may be sufficient to remotely locate the antenna, photomixer, and laser IF modulator. 
   It is an object of the present invention to provide a very wideband terahertz-range communications system. 
   It is an object of the present invention to utilize a photomixer in implementing a terahertz communications system. 
   It is an advantage of the present invention to utilize lasers and low-loss fiber optic components to aid in implementing a terahertz communications system. 
   It is an advantage of the present invention to provide continuous-tuning in the terahertz frequency band. 
   It is a feature of the present to be able to locate terahertz converters and other RF components at large distances from the remainder of the communications system due to the use of low-loss fiber optic cables. 
   It is a feature of the present invention to replace tunable frequency synthesizers for the terahertz range that are difficult or impossible to implement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: 
       FIG. 1  is a graph showing atmospheric propagation in the terahertz frequency range; 
       FIG. 2  is a block diagram of a conventional search receiver architecture for the terahertz frequency range; 
       FIG. 3  is a diagram of a photomixer and antenna; 
       FIG. 4  is a block diagram of a terahertz receiver of the present invention using the photomixer of  FIG. 3 ; 
       FIG. 5  is a block diagram of another embodiment of a terahertz receiver of the present invention; 
       FIG. 6  is a block diagram of a terahertz range transmitter using the photomixer of  FIG. 3 ; and 
       FIG. 7  is a block diagram of a terahertz range transceiver using the photomixer of  FIG. 3 . 
   

   DETAILED DESCRIPTION 
   The implementation of a conventional receiver  200  for the terahertz frequency range is following the current architecture of millimeter wave receivers, that is, a group of narrowband block converters  210   a–d  in parallel as shown in  FIG. 2 . Due to component technologies, primarily the limited frequency response of each semiconductor process or structure, only a 10–20 GHz bandwidth is typically attainable. Tunable frequency synthesizers spanning any significant frequency range are difficult or impossible to implement to support a superheterodyne converter architecture. Thus a typical surveillance system consists of a bank of individual block converters  210   a–d , each spanning 10–20 GHz using a fixed local oscillator (not shown), and typically each with its own antenna  215   a–d  to eliminate a high-loss microwave switching subsystem. These block converters  210   a–d  typically convert down to a 2–20 GHz range and are selected by a switch  217 . A tunable receiver  220  processes and/or demodulates the incoming signals for useful information. In the case of a search receiver system, a fast Fourier transform (FFT) (not shown) operates on a wideband output from the tunable receiver  220 . 
   A more interesting approach to implementing a terahertz communications system comes from recognizing that the terahertz band is only an order of magnitude in frequency below the mid-infrared optical band as shown in  FIG. 1 , suggesting that an electro optical implementation may be a more capable, albeit innovative, approach to implementing a terahertz receiver. Recent work on the heterodyne conversion of signals between the optical and microwave domains is particularly relevant to the present invention. 
   The present invention builds upon experimental work done on optical heterodyning and frequency conversion as described in U.S. Pat. No. 5,663,639 incorporated herein by reference. A pair of lasers separated in frequency by a desired local oscillator (LO) frequency is applied to a photodiode, called a photomixer  305  shown in  FIG. 3 , whose non-linearity then generates a signal equal to their difference.  FIG. 3  shows an integrated spiral antenna  310  and the photomixer  305  used to generate and radiate signals between approximately 100 GHz and 1 THz using a laser pair illuminating the photomixer  305 . 
   The present invention advances the previous experimental work toward an ultrawideband RF receiver  400  shown in  FIG. 4 . An integrated circuit  410  includes an antenna  405 , the photomixer  305 , and an RF mixer  407  to form a block converter. The integrated circuit  410  allows a local oscillator signal generated by the photomixer  305  to be directly coupled to the on-chip Schottky-diode mixer circuit  407 . The wideband log spiral antenna  405  covers the 20–120 GHz range and can be expanded to 20–600 GHz. Other types of antennas known in the art may also be used. An IF output from the integrated circuit block converter  410  is applied to the 2–20 GHz receiver  220  for fine resolution tuning and further processing. 
   The photomixer  305  local oscillator is pumped by a fixed continuous wave (CW) laser  415  with a constant output power and wavelength λ 1 , and a free-running variable laser  420  whose wavelength λ 2  is tunable. The fixed laser  415  and variable laser  420  illuminate the photomixer through fiber optic cables  425 . The tunability of the variable laser  420  may be realized by a number of methods such as varying the laser temperature under electronic control. A search receiver application requires sweeping the laser  420  between its lower and upper temperature (and wavelength) limits, either as a sawtooth or triangular waveform pattern. The instantaneous frequency of the photomixer  305  local oscillator is determined by subtracting the fixed wavelength λ 1  of the fixed laser  415  from the wavelength λ 2  of the variable laser  420 , λ 2 -λ 1 . 
   The variable laser  420  may be phase-locked to allow precise stable tuning steps. Phase locking will offer digital frequency tuning accuracy and repeatability. Phase locking may be accomplished by any number of known means in the art. 
   Two fiber optic cables  425  are shown in  FIG. 4 . The two laser outputs may be combined in an optical combiner  530  and a single fiber optical cable  425  may be connected to the combiner output to provide the combined laser beams to the photomixer  305  as shown in  FIG. 5 . 
   An analog laser IF modulator  435  may be used to modulate a laser beam and another fiber optic cable  425  to conduct the laser beam to a laser IF demodulator  440  that then demodulates the laser beam and passes it to the tunable IF receiver  220  for further processing. The fiber optic cable  425  connected to the IF modulator  435  may be combined with the single fiber optic cable  425  that provides the combined laser beam to the photomixer  305  using optical methods and devices known in the art. Conventional coaxial cable interconnect means may also be used to conduct the IF signal to the tunable receiver  220 . The laser IF modulator  435  and demodulator  440  are known in the art and are commercially available devices. 
   The system architecture  400  shown in  FIG. 4  of the present invention is that of a superheterodyne receiver. A characteristic of this architecture is the lack of image rejection due to the lack of selectivity between the antenna  405  and mixer  407 . For the search receiver application a fast Fourier transform  450  eliminates image responses mathematically by performing pattern matching and masking using FFT data processing while continuously scanning a desired band. More specifically, high side and low side scans of a desired center frequency are compared, and redundancies removed by masking a redundant spectral product. Due to the relatively uncrowded nature of the millimeter-wave and terahertz spectrum, this technique will result in very useful performance for a wideband search system. 
   The search receiver  400  of  FIG. 4  may be utilized over a frequency range of typically 20–120 GHz and expanded to higher frequencies by implementation of appropriate semiconductor processes. 
   The integrated circuit  410  combines the planar antenna  405 , RF mixer  407 , photomixer  305 , and an IF output network (not shown) in a single package. Each of these devices may typically be fabricated from InGaAs lattice-matched to an InP substrate as an example. 
   It should be noted that the use of lasers to generate the local oscillator signal allows the use of the conventional fiberoptic cables  425  to deliver the pump power to the photomixer  305  from remote tunable and fixed lasers and the IF signal to the tunable receiver  220 . This permits the millimeter-wave and terahertz converter  410  to be located hundreds or even thousands of feet of cable run away from a central collection location, thanks to the use of low-loss fiberoptic cable versus traditional cable or waveguide whose loss increases exponentially with frequency for both the local oscillator signals and the IF output. The capability to remotely locate converters is unknown with any current technology at terahertz frequencies. 
   The present invention results in a novel terahertz-range signal receiver  400  architecture that is capable of fast-tuning and continuous signal coverage from microwave up through the terahertz range. This receiver  400  is well suited to various applications in electronic warfare. The receiver  400  of the present invention is also suitable for applications in test equipment as well as fiberoptic communications networks and wireless communications systems. Yet another application may be a radar receiver with extremely fine resolution. 
   Another embodiment of a terahertz receiver  500  using the photomixer  305  is shown in  FIG. 5 . In this embodiment converter  510  comprises the antenna  405 , an image reject filter  507  connected to the antenna, a low noise amplifier (LNA)  508 , and the photomixer  305 . The RF mixer  407  of  FIG. 4  is eliminated and mixing of a received RF signal with the local oscillator generated from the difference frequencies of the two laser beams also occurs in the photomixer  305 . The image reject filter  507  may be a fixed or tunable filter depending on application requirements. The image reject filter  507  may also be eliminated and images processed by the FFT  450  as in  FIG. 4 . 
   In the embodiment shown in  FIG. 5 , fixed laser  415  at λ 1  and variable laser  420  at λ 2  have their outputs combined in an optical combiner  530  and a single fiber optic cable  425  conducts the laser beams to illuminate the photomixer  305 . The laser IF modulator  435 , IF fiber optic cable  425 , and laser IF demodulator  440  may again be used as in  FIG. 4  or conventional coaxial cables may be used. Alternately, IF filters and amplifiers (not shown) may be used. Using fiber optic cables  425  again allows remotely locating the converter  510  at great distances from the remainder of the receiver. 
   A terahertz-range transmitter  600  using the photomixer  305  is shown in  FIG. 6 . A laser  615  may be directly modulated by a desired form of modulation, depending on communications system requirements to output a modulated laser signal at a first frequency. The modulation may be amplitude, wavelength (frequency), or phase modulation. Modulation of the laser  615  is accomplished by conventional means. The laser  615  output may be modulated by an optional external modulator  625 . The same types of modulation may again be utilized. The laser  615  may be modulated directly and the external modulator  625  may also be used to enable modulating with two modulating signals. 
   The modulated laser signal is connected with fiber optic cables  425  to the optical combiner or multiplexer  530  as in the receiver  500 . Another laser  420  provides a second laser signal over another fiber optic cable  425  to the combiner  530 . This second laser  420  may be varied in frequency to tune the transmitter  600  to a desired frequency. The combined laser beams from the fiber optic cable  425  is connected to the photomixer  305  to generate a transmit frequency in the terahertz frequency range where the transmit frequency is the difference between the two laser frequencies λ 2 -λ 1  as in the terahertz receiver  400 . The antenna  310  radiates the modulated terahertz transmit signal. 
   The antenna  310  may be eliminated and the output of the photomixer  305  may be coupled to a power amplifier (not shown) to increase the power level and then radiated by an external antenna (not shown). The antenna  310  may be used to radiate the modulated signal to a spatial power combining power amplifier known in the art to obtain a high-power output signal. A spatial power combiner is an array of unit cells forming a grid. Each cell comprises an input antenna, a monolithic microwave integrated circuit (MMIC) amplifier, and an output antenna. 
   As with the terahertz receivers  400  and  500 , the transmitter  600  may have the photomixer  305 , antenna  310 , and any associated power amplifiers remotely located from the laser sources  515  and  520 . The use of a long fiber optic cable having low losses enables such an installation. Long coaxial cables have prohibitive losses in the terahertz frequency range. 
   A terahertz transceiver  700  employing the photomixer  305  is shown in  FIG. 7 . The terahertz transceiver  700  is a homodyne optical full-duplex transceiver. In an RF module  710  receiver portions of the transceiver  700  include the receive antenna  405 , optional image reject filter  507 , and LNA  508  as in the superheterodyne receiver  500  of  FIG. 5 . A transmit antenna  705 , optional transmit filter  707 , and power amplifier  708  form portions of the transmitter. A combiner/splitter  709  or circulator provides the input to the power amplifier  708  from the photomixer  305  and provides the received signals to the photomixer  305  and the rest of the circuitry. A high-pass filter  715  filters the receiver and transmitter signals to and from the photomixer  305 . The high-pass filter  715  passes the local oscillator frequency and the received signal while blocking the baseband signal. The fiber optic cable  425  provides the two laser beam inputs to the photomixer  305  from the optical combiner  530  as in  FIGS. 5 and 6 . Laser  615  providing a laser beam at wavelength λ 1  is modulated as in  FIG. 6 . Laser  420  providing a laser beam at wavelength λ 2  may be tunable to vary the transceiver  700  frequency. The receiver output from the photomixer  305  is passed through a low-pass filter  720  for further baseband processing. The low-pass filter blocks the local oscillator frequency and the received signal but passes the baseband signal. 
   The photomixer  305  generates the LO (local oscillator) at λ 2 -λ 1  as in the receivers  400  and  500  and transmitter  600 . For this example laser  615  is assumed to be frequency modulated thereby varying its wavelength (frequency) as a function of time λ 1 (t). The transmitted signal then is at λ 2 -λ 1 (t). The receiver portion of transceiver  700  operates as a homodyne or direct conversion receiver. The received signal is mixed with the LO at λ 2 -λ 1 (t) resulting in a receive signal at baseband. The baseband signal contains a frequency modulated component due to being mixed with the LO signal having the transmit frequency modulated component λ 1 (t). The modulation may be cancelled by applying an out-of-phase modulating signal to the receiver baseband signal. 
   It is believed that the electro optic microwave communications system of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.