Abstract:
An apparatus comprising a fiber optical loop for conducting a first and a second pulse having a corresponding first and second wavelength, a first splitter for separating the first and second light pulses in the optical loop into a first and second light path to introduce a predetermined time delay between the first and second light pulses, a coupler for tapping a replica of the pair of light pulses from the loop, an auto-correlation module, coupled to the coupler, for correlating the replica of the pair of light pulses with each other to produce a set of data points comprising a plurality of multiplied and correlated pair of pulses and a transform module, coupled to the auto-correlation module, for transforming the data points into a channelized frequency spectrum.

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 analyzing the spectrum of radio-frequency signals using a fiber optic recirculation loop. 
     BACKGROUND OF THE INVENTION 
     Typically, public radio-frequency (RF) communications are transmitted at a preconfigured 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 a communication is transmitted across various frequencies, i.e., frequency hopping is employed, in the form of short RF pulses where each broadcast is 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 a method and apparatus for analyzing the spectrum of radio-frequency signals using a fiber optic recirculation loop. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to an apparatus comprising a fiber optical loop for conducting a first and second light, generated by a first and second light source, modulated by a radio frequency signal by a modulator to create a first and second pulse with corresponding first and second wavelengths, a first splitter for separating the first and second pulses in the optical loop into a first and second path for a small portion of the loop to introduce a predetermined time delay between the first and second pulse, a coupler for tapping a replica of the pair of pulses from the loop and an auto-correlation module, coupled to the coupler, for correlating the replica of the pair of pulses with each other. 
     Another embodiment of the present invention is directed to a method determining a channelized frequency spectrum from an RF signal comprising conducting, in a fiber optical loop, a first and second light generated by a first and second light source, modulating, using a modulator, the first and second light by a radio frequency signal to create a first and second pulse with corresponding first and second wavelengths, splitting, using a first splitter, the first and second pulses in the optical loop into a first and second path for a small portion of the loop to introduce a predetermined time delay between the first and second pulse, tapping, using a coupler, a replica of the pair of pulses from the loop, correlating, using an auto-correlation module, coupled to the coupler, the replica of the pair of pulses with each other and performing a Fast-Fourier Transform (FFT), using an FFT module, on a plurality of the correlated pair of pulses, generating a channelized 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 fiber optic recirculation loop; and 
         FIG. 2  is a flow diagram of method for detecting radio-frequency signals using an optical loop. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention comprise a method and apparatus detecting radio-frequency signals using a fiber optic recirculation loop, by modulating an input RF signal using two lasers of differing wavelength, and introducing a time delay between the lasers to auto-correlate the two time-delayed lasers. Light from the first and second laser with differing wavelengths are modulated using the received RF input signal, which is comprised of multiple frequencies. The modulated lights are converted into a pair of pulses of light. The pulse circulates through the optical loop a finite number of times. A split path in the loop with a time adjuster allows for introducing a customizable delay between each pulse of light, irrespective of the natural delay caused by the difference of travel time due to the difference of wavelength in the pair of pulses. 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 two RF signals; the two RF signals 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 a fiber optic recirculation loop. The apparatus  100  comprises a first laser  102 , a second laser  104 , a combiner  106 , a first polarizer  108 , an optical modulator  110 , a second polarizer  112 , an optical switch  114 , an electronic time control  133 , a 2-by-2 coupler  116 , a first isolator  118 , a wavelength division multiplex (WDM)  120 , an amplifier  122 , a second isolator  124 , a splitter  126 , a filter  128 , a second combiner  130 , an optional phase shifter  132 , an electronic time control  133 , a switch  134 , a second polarizer  136 , a second splitter  138 , a first photodetector  142 , a second photodetector  144 , a second combiner  146  and a square-law RF detector  148  which form a an auto-correlation circuit (e.g., an RF electronic multiplexer)  145 , a second filter  150 , an analog to digital converter (ADC), and a computer  154 . The computer  154  comprises a processor  159  and memory  157 . The memory  157  comprises an Fast-Fourier transform module  156  and stores data points  160 . 
     According to an exemplary embodiment of the present invention, the apparatus  100  processes an RF input signal  101  and produces frequency spectrum  156  representing information carried by the RF input signal  101 . The RF input signal  101  is an RF signal and the first and second laser  102  and  104  are carrier wave light signals. 
     The RF input signal  101  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 lasers  102  and  104  provide laser light of wavelength λ 1  and λ 2 , respectively. The combiner  106  combines the laser lights from lasers  102  and  104 . According to an exemplary embodiment, the combiner  106  is a wavelength division multiplex (WDM) and the frequency difference between λ 1  and λ 2  is much larger than the RF microwave operation frequencies and bandwidth. In some embodiments, the laser light from lasers  102  and  104  is infrared light and the difference in wavelength λ 1  and λ 2  is approximately 100 GHz. The combined light output from the combiner  106  is optionally coupled to a polarizer  108  to polarize the light. According to an exemplary embodiment, the polarizer  108  is a fiber polarizer. The combiner  106  (or optionally the polarizer  108 ) transmits the combined light to the optical modulator  110  to modulate the laser lights by the input RF signal. The polarizer  108  may be located before the optical modulator  110  or before the optical modulator  110  according to various embodiments of the present invention. 
     The optical modulator  110  modulates light from the laser  102  and  104  with the RF input signal  101 , creating an RF modulated light wave. According to an exemplary embodiment, the lasers  102  and  104  are continuous wave lasers. Also according to an exemplary embodiment of the present invention, the optical modulator  110  is a Mach Zehnder modulator. The optical switch  114  (e.g., an optical gate switch) converts the modulated light into a pair of RF modulated optical pulse. Optionally, the switch  114  is a 1×2 switch in which the first output carries the pulses to a 2×2 optical coupler  116 . The second output of the switch  114  can be used to connect to another optical switch for time division multiplex cascading to additional recirculation loops to process any signals received within a large time window such as continuous wave (CW) signals as opposed to pulsed RF signals. 
     Once the switch  114  generates the optical pulses, the pulses enter the coupler  116 . According to an exemplary embodiment, the coupler has a coupling ratio range from 50%:50% to 1%:99%. Once the pulses enters the switch  114 , the switch  114  closes so further pulses cannot enter the loop  105  causing distortion and noise. 
     The optical recirculation loop  105  is formed by connecting one output of the coupler  116 , which according to an exemplary embodiment is a 2×2 coupler, to the second input of the same coupler with several optical components in the loop. According to exemplary embodiments of the present invention, the loop length generally does not affect the ability to tap out replica pulse pairs, however loops with length less than one pulse width will cause overlap and distort results. The loop  105  may be a dispersion shifted fiber where there is non-dispersion around the operating wavelength of the lights. 
     The optical loop contains an isolator  118  which isolates the pulses. An optional WDM  120  combines a third wavelength by forming a third loop between the WDM splitter  126  and  120 . The amplifier  122  amplifies the isolated pulse pairs from isolator  118 . Optionally, isolator  124  is placed in the loop after the amplifier  122  to allow a continuous wavelength (CW) light with the third wavelength be amplified so to keep the amplifier working in a steady state instead an unstable state when there are only short pulse to be amplified. This will further reduce any noise in the pulse pairs. According to an exemplary embodiment of the present invention, the optical amplifier  122  is an Erbium doped fiber amplifier (EDFA), though the present invention does not limit the type of optical amplifier used. 
     The optical amplifier  122  amplifies the pulses to compensate for the coupling loss. The isolator  118  is coupled to the loop  105  to prevent any light going in the reverse direction from causing interference in the loop  105 . The pulse pairs are then transmitted to a splitter  126 . According to an exemplary embodiment, the splitter  126  is a WDM splitter which routes the pulse pairs into a first path  127  and a second path  131  according to their wavelength λ 1  and λ 2  to introduce a preconfigured delay between the transmission of the first and second pulse. This allows for auto-correlation of the two pulses later on. 
     According to one embodiment of the present invention, the EDFA  122  can be replaced by two EDFAs placing in each path  127  and  131 . According to an alternate embodiment, there is a gain control attenuators in each path  127  and  131  in place of the two EDFAs to balance the intensity/power of the first and second modulated signal. The WDM splitter  126  is, according to an exemplary embodiment, implemented a two wavelength filters which filter for optical noise produced by the amplifier  122 . 
     Filter  128  is an optional filter for removing any further noise in the pulse traversing the first path  127 . The adjuster  120  is an optical path length adjuster for adjusting relative travelling time between the two differing wavelength pulses, i.e., the path length adjuster will increase or decrease the time it takes for a pulse travelling the first path  127  to travel through the path  127 . A combiner  130  combines the two pulses and couples the pulses to a switch  134 , which according to an exemplary embodiment is a high speed optical switch used as a time domain filter to filter out light noise outside of the switching window. The phase shifter  132  is optionally added before the switch  134  to introduce a random phase shift to prevent the residue CW light noise because resonant noise (lasing). The output of switch  134  is connected to a polarizer  136  for reducing polarization dependent effects inherent in the fiber of the loop to change the coupling ratio for the coupler  116  and then back to an input of the coupler  116  to close the loop  105 . 
     The electronic time control circuit  133  controls the functionality of the switch  134 , by synchronizing with control of the switch  114 . The coupler  116  outputs a replica pulse pair at its second output after each loop cycle. The splitter  138  (e.g., a WDM splitter) splits the replicated pulse pairs into individual pulses. The photodetectors  142  and  144  each convert the RF modulated optical pulses into RF modulated electronic pulse signals. 
     For n passes through the loop  105 , the nth pulse pair replica will have a time delay of nΔt between the two pulses due to the first path  127  having the adjuster  129  for adjusting travel time of one of the pulses, where Δt=ΔL/c. ΔL is the effective optical path difference of the two pulse wavelengths in one loop cycle. The wavelengths of the lasers  102  and  104  are customizable. Optionally, an RF phase shifter is connected to the output of the photodetector  142  to set an initial phase adjustment, for example, φ, between the two pulses of each pair. 
     If a single RF frequency ω signal is introduced at the input, 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 pulse signal amplitudes, and t n  is the time that has passed as of the n th  loop cycle. In an exemplary embodiment, a negative feedback control is provided for a servo  140  from the combiner  146  to control the gain of the amplifier  122  in the loop  105 , preventing over and under amplification and compensating for drift in the loop  105 . 
     The output RF electronic pulse signals from the photodetectors  142  and  144  are coupled to a RF electronic multiplexer circuit  145  to multiply the pulse pairs together. This is also known as an auto-correlation module. The multiplexer  145  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  101 . According to an exemplary embodiment, the circuit  145  comprises a 2×1 RF combiner  146  connected to a square-law RF detector  148  to produce the multiplication product for the correlation process. 
     In an exemplary embodiment, the detector  148  is a square law RF detector such as a crystal detector (Low Barrier Schottky diode) or an RF mixer. The output of the multiplexer  145  is coupled to an optional RF filter  150  which filters the high band frequencies, i.e., a low pass filter, yielding just a baseband signal cos(ωnΔt+φ) and a DC signal. A DC block may optionally be used to filter out the DC signal. The baseband signal is coupled to an ADC  152  such that each replicated pulse pair yields a data point for autocorrelation with a different delay between the pulse pair. 
     These points are stored in memory  157  as data points  160 . The computer  154  (e.g., a fully programmable gate array, or the like) contains a Fast Fourier transform (FFT) module  156  in its memory  157  which performs a Fourier transformation to transform the series of pulse pairs from time domain to frequency domain, thereby obtaining the RF spectrum of the input RF pulse  101 . In an exemplary embodiment, the memory  157  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  156  performs an FFT on the data points from the ADC  152  to generate a channelized frequency spectrum. The FFT module  156  converts the data points from the time domain to the frequency-squared domain, from which the frequency spectrum data  1158  is obtained using signal processing techniques known to those of ordinary skill in the art. 
     According to other exemplary embodiments of the present invention, an optical path length random perturbation device  132  such as an optical phase shifter or fiber stretcher is inserted in the loop  105  to reduce the resonance effect on the noise signal in the loop. An optional ring laser loop is added in the loop  105  to stabilize the optical amplifier  122  by allowing a third wavelength be constantly amplified by the EDFA and cause lasing in continuous wave (CW) mode. In other embodiments, laser  102  and laser  104  are replaced with laser transmitters with wavelength λ 1  and λ 2 . The RF input signal  101  is split and used to modulate the two laser transmitters. 
     According to other exemplary embodiments of the present invention, photodetectors  142  and  144 , the splitter  138  and the combiner  146  are replaced by one photodetector and an amplifier combination at the output of lop  105  at the 2×2 coupler  116 . According to this embodiment, optical pulse monitoring by the gain control serve  140  must be tapped from inside the loop as opposed to from the combiner  146 . 
       FIG. 2  is a flow diagram of method  200  for detecting radio-frequency signals using an optical loop. The method begins at step  202  and proceeds to step  204 , where a first and second light from a laser is modulated by an RF input signal using RF modulator  110 . At step  206 , the switch  108  generates a pair of pulses of the modulated optical signal with a first and second wavelength based on the wavelength of the first and second light. At step  208 , a replica of the pulse is tapped from the loop  105  for processing, while the remaining portion of the pulse re-circulates in the loop  105 . 
     At step  210 , first and second RF signals are generated from the pulse pair and then auto-correlated at step  212 . The two RF signals have a time delay between them due to the first path having an adjuster  129  embedded in it, for reducing or increasing the time a pulse takes to travel the first path  127 . At step  214 , data points are generated by the ADC  108  to digitize the auto-correlation and the digitized data points are stored in memory  132 . At step  216 , 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  218 . At step  220 , 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  222 . 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.