Patent Publication Number: US-8532486-B2

Title: Method and apparatus for detecting radio-frequency signals using a dispersive fiber optical loop

Description:
GOVERNMENT INTEREST 
     Governmental Interest—The invention described herein may be manufactured, used and licensed by or for the U.S. Government. 
    
    
     FIELD OF INVENTION 
     Embodiments of the present invention generally relate to radio-frequency communications and, more particularly, to a method and apparatus for detecting radio-frequency signals using a dispersive fiber optical loop. 
     BACKGROUND OF THE INVENTION 
     Typically, public radio-frequency (RF) communications are transmitted at a set frequency so that a receiver can tune to the particular frequency and receive the communications. In contrast, private communications are transmitted across multiple frequencies (e.g., using frequency hopping and/or spread spectrum techniques). In some instances, these private communications need to be captured by unintended receivers, e.g., law enforcement agencies, military organizations and the like. However, difficulties arise when the communications are transmitted across various frequencies, i.e., frequency hopping is employed, in the form of short RF pulses each broadcast on a different frequency. 
     Without knowing the frequency hopping pattern, a receiver must attempt to capture all signals in the relevant band. Typically, all the signals within the band are digitized and then processed using a very high speed digital signal processing (DSP) system. Such high speed DSP systems are very costly to manufacture, operate and maintain. In some instances, the band of interest is divided into sub-bands and each sub-band is digitized and processed in a corresponding DSP. Such sub-band channelization enables many signals to be quickly processed in parallel using less expensive DSP circuits (i.e., lower speed circuits). However, even a channelized, broad band receiver is very expensive to manufacture, operate and maintain. 
     Recently, optical systems have found use in broad band signal processing wherein the received RF signals are used to modulate a light signal and the light signal is processed using optical signal processing. Such techniques, unfortunately, are prone to noise and system instability. 
     Therefore, there is a need in the art for an improved method and apparatus for detecting radio-frequency signals using optical techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to an apparatus comprising a dispersive fiber optical loop for delaying a first and second sideband, relative to each other, of a laser modulated by a radio frequency signal to create a pair of pulses. A coupler taps a replica of the pair of pulses from the loop. An auto-correlation module, coupled to the coupler, correlates the replica of the pair of pulses with each other producing data points. Each transit around the loop produces an additional replica of the pair of pulses, with an augmented delay between them. A Fast Fourier Transform (FFT) module performs an FFT on the data points to obtain a channelized frequency spectrum representing the radio frequency signal. 
     Another embodiment of the present invention is directed to a method modulating a light with a RF input signal, to produce a modulated light with a first and second sideband, generating a pulse from the modulated light, inputting the pulse into a dispersive fiber optical loop to generate a delay between the first and second sideband, relative to each other, tapping a plurality of time delayed replicas of the pulse from the loop, photo-detecting the time delayed replicas to generate a plurality of RF signal pairs, correlating the plurality of RF signal pairs by correlating each first signal in the pair with a second signal in the pair, generating and storing data points based on the correlating, performing a Fast Fourier Transform on the plurality of stored data points to generate a frequency spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a block diagram of an apparatus for detecting radio-frequency signals using a dispersive fiber optical loop; and 
         FIG. 2  is a flow diagram of method for detecting radio-frequency signals using a dispersive fiber optical loop. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention comprise a method and apparatus for detecting radio-frequency signals using a dispersive fiber optical loop. Light from a laser is modulated using a received RF signal, which is comprised of multiple frequencies, producing two sidebands. The modulated light is converted into a pulse of light. The pulse circulates through the dispersive optical loop a finite number of times. The dispersive nature of the loop causes the pulse containing both sideband frequencies to propagate through the loop at different speeds for each sideband. As such, after one pass through the loop, there are two pulses each representing a different sideband. For every cycle the pulses take through the fiber, a replica pulse pair is “tapped” from the loop. The tapped pulse pair is photo-detected to create an RF signal; the sidebands in the pulse pair are correlated with each other within an RF mixer, filtered, and then digitized. Once the loop has run multiple times, a Fast Fourier transform (FFT) is performed on the accumulated digitized signals (data points) to generate a channelized frequency spectrum representing information carried by the received RF signals. 
       FIG. 1  is a block diagram of an apparatus  100  for detecting radio-frequency signals using an optical loop. The apparatus  100  comprises a laser  104 , an optical modulator  106 , a gate switch  108 , a 2×1 switch  110 , a 1×2 coupler  112 , a first isolator  114 , an optical amplifier  116 , an optical filter  118 , a second isolator  120 , a photo-detector  122 , an auto-correlation module  123  and an analog-to-digital converter (ADC)  128 . The apparatus  100  further comprises a controller  130  with memory  132  and an FFT module  134 . The apparatus  100  processes an RF input signal  102  and produces a frequency spectrum  136  representing information carried by the RF input signal  102 . The RF input signal  102  is an RF signal and the laser  104  is a carrier wave light signal. 
     The RF input signal  102  may be received via a conventional RF front end (not shown) comprising, for example, an antenna, low noise amplifier and various filters. The choice of components and arrangement depends upon the nature of the signals that are to be processed. The selection of components and their arrangement is a design choice capable of being made by anyone with skill in the RF communication arts. 
     The optical modulator  106  frequency modulates light from the laser  104  with the RF input signal  102 , creating an RF modulated light wave. According to an exemplary embodiment, the laser  104  is a continuous wave laser providing light of frequency f 0 . Also according to an exemplary embodiment of the present invention, the optical modulator is a Mach Zehnder modulator. The sidebands created by the optical modulator are separated from the light frequency by the frequencies of the RF signals. For example, if an RF input signal has a frequency of 1 GHz, sidebands would be located at f 0 +1 GHz and f 0 -1 GHz. 
     The optical gate switch  108  converts the modulated light into an optical pulse. Optionally, after the switch  108  and before entry into the loop at switch  110 , a 1×2 coupler is used to send the pulse signal  111  to another loop. Loops can be cascaded so that pulse signal  111  is multiplexed in the time domain to many copies of apparatus  100  for processing longer pulses. Alternatively, a 1×n switch is coupled to the gate switch  108  with each output of the 1×n switch connected a recirculating fiber loop of differing lengths, allowing an operator of apparatus  100  to select a desired bandwidth for the system. 
     Once the gate switch  108  generates an optical pulse  111 , the pulse  111  enters the beginning portion of the fiber optic recirculating dispersion loop  109 . In an exemplary embodiment, the fiber optical dispersion loop  109  is 10 km in length and is fabricated of the SMF-28® ULL fiber from Corning Incorporated of Corning, N.Y. The loop  109  is recirculating because the loop  109  enables the pulse  111  to circle around and re-enter the switch  110  to introduce a time delay caused by dispersive properties of the loop  109 . Once the pulse  111  enters the switch  110 , the switch  110  closes so further pulses cannot enter the loop  109  causing distortion and noise. A single pulse  111  enters the loop. This single pulse  111  comprises frequency components of both side bands. The dispersive nature of the loop causes each side band to propagate through the loop at a different speed. Consequently, a delay is created between the side bands that creates the two pulses separated by a delay, n Δt, where n is the number of times the pulse has circled the loop and Δt is the time delay formed during a cycle through the loop. According to another exemplary embodiment of the present invention, the gate switch  108  is coupled to a plurality of cascaded fiber optical loops (not shown) for processing longer pulses. In one embodiment, each of these loops is of a different length, determining the frequency band the loops can process. 
     The loop  109  comprises a coupler  112  that taps a replica of the pulse  111  for further processing, discussed below. In another embodiment of the present invention, the switch  110  and the coupler  112  are combined as a 2×2 coupler, with a first input for the high contrast gate switch  108 , a second input for the loop  109 , a first output, to tap replica pulses and a second output coupled to the isolator  114 . 
     The pulse signal  111  that remains suffers a signal loss (i.e., coupling loss), so it is coupled to an isolator  114  and an optical amplifier  116  in the loop  109 . According to an exemplary embodiment of the present invention, the optical amplifier  116  is an Erbium doped fiber amplifier. The optical amplifier  116  amplifies the pulse  111  to compensate for the coupling loss. Often, an optical amplifier  116  produces reflective pulses, thus the isolator  114  is coupled to the loop  109  prevents these reflections from causing interference in the loop  109 . The fiber optical loop  109  is then coupled to an optical filter  118  at the laser  104  wavelength to filter any noise generated by the amplifier  116 . The loop  109  is coupled to the isolator  120  for isolating any reflections from the switch  110 . In an exemplary embodiment, a polarization rotator is inserted before the coupler  112  to maximize coupling efficiency. In addition, an optional second polarization rotator is used in the loop  109  to allow minimization of polarization dependent effects by not allowing devices in the loop to have polarization dependent loss or gain, i.e., the polarization rotators randomize the polarization of the light propagating in the loop  109 . 
     The pulse  111  (which becomes a pulse pair while in the loop) travels around the loop multiple times, with a replica of the pulse  111  tapped out each time. Due to the dispersion in the fiber, one sideband of the pulse  111  travels slower than the other sideband. The difference in travel time is denoted by Δt, so each loop introduces an additive delay nΔt where n is the current loop iteration, between the sidebands in each successive pulse pair replica. Regular optical fiber causes the spreading apart of optical pulses, i.e., dispersion, by approximately wavelength λ. For the nth pulse pair, the Δt between the two pulses will be nΔt, as discussed above, where 
     
       
         
           
             
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     Here, L is the loop length, λ 1  and λ 2  are the modulation sideband wavelengths and S 0  is a constant from the dispersion relation intrinsic to the optical fiber. For the Corning&#39;s SMF-28® ULL fiber, S o  is 0.092 ps/(nm 2  km), although other dispersive fibers can be used. 
     The tapped pulse pair replicas are coupled to an optional optical amplifier  121  that amplifies the signal for the photodetector  122 . The photodetector  122  converts the optical pulse sidebands to a pair of RF pulses. The photo-detector  122  is coupled to an auto-correlation module  123 . In one embodiment, auto-correlation module  123  comprises an optional RF amplifier  124  that amplifies the RF signal created by the photodetector  122 , an RF mixing element  125  and an RF filter  126 . For the n th  pulse pair, the two pulse signals are represented as A cos(ωt n ) and B cos(ω(t n +nΔt)+φ, where A and B are the RF sideband signal amplitudes, ω is the angular frequency of the RF input signal and t n  is the time. In an exemplary embodiment, a portion of output power from the loop  109  before going to the photodetector  122  is used as a negative feedback control for a servo for altering amplification of the amplifier  116 , preventing over and under amplification and compensating for drift in the loop  109 . 
     The RF signals are sent to an RF mixing element  125  to multiply together each pulse pair. This is known as an auto-correlation function. In an exemplary embodiment, a square law detector is used as the RF mixing element  125 . The RF mixing element  125  integrates many pulses over time. The pulse pairs are auto-correlated to help in finding the presence of repeating patterns and periodic signals in the RF input  102 . An optional RF filter  126  filters the high band frequencies, i.e., a low pass filter, yielding just a baseband signal cos(ωnΔt+φ) and the DC signal. A DC block may be used to filter out the DC signal. The baseband signal is coupled to an ADC  128  such that each replicated pulse pair yields a data point for autocorrelation with a different delay between the sidebands. These points are stored in memory  132  as data points  135 . 
     A controller  130  synchronizes the opening and closing of the gate switch  108 , the switch  110  and the ADC  129  as well as routing to any additional loops mentioned in the optional configurations. The controller  130  comprises memory  132  and a processor  131 , coupled to the memory  132 . The memory  132  stores an FFT module  134  and data points  135 . In an exemplary embodiment, the memory  132  may include one or more of the following: random access memory, read only memory, magneto-resistive read/write memory, optical read/write memory, cache memory, magnetic read/write memory, and the like, as well as signal-bearing media, not including non-transitory signals such as carrier waves and the like. 
     The FFT module performs an FFT on the data points from the ADC  128  to generate a channelized frequency spectrum. In the present invention Δt is not constant and is dependent on the frequency of the RF input  102 . The loop  109  enables the apparatus to create a linear dependency between the Δt and RF input  102  frequency. In this manner, the baseband signal cos(ωnΔt+φ) can be rewritten cos(ω 2 nC+φ), where C is a constant dependent on the dispersion properties of the loop  109 . The FFT module  134  converts data points  135  from the time domain to the frequency-squared domain, from which the frequency data is obtained using well known signal processing techniques. 
       FIG. 2  is a flow diagram of method  200  for detecting radio-frequency signals using a dispersive optical loop. The method begins at step  202  and proceeds to step  204 , where a light from a laser is modulated by an RF input signal using RF modulator  106 . At step  206 , the switch  108  generates a pulse of the modulated optical signal with a first and second sideband. At step  208 , a replica of the pulse is tapped from the loop, while the remaining portion of the pulse re-circulates in the loop. 
     At step  210 , the sideband pulse replica pairs are photo-detected to generate RF signals and then correlated with each other at step  212 . The two sidebands have a time delay between them due to the dispersion in the fiber optical loop. At step  214 , data points are generated by the ADC  108  to digitize the auto-correlation. The digitized data points are stored at step  216  in memory  132 . At step  218 , the method determines whether N points have been stored. If N points have not been stored, the method returns to step  208  to tap more pulses from the loop  109 . If N points have been stored, the FFT module  134  performs a FFT on the N points to generate a frequency domain spectrum at step  220 . At step  222 , it is determined whether a new pulse should enter the loop. If so, the method proceeds to step  204  of modulating light from a laser with an RF input. If there are no more pulses, the method ends at step  224 . According to an exemplary embodiments, N=500, 1000 or the like. 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as may be suited to the particular use contemplated. 
     Various elements, devices, modules and circuits are described above in associated with their respective functions. These elements, devices, modules and circuits are considered means for performing their respective functions as described herein. While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.