Method and apparatus for analyzing the spectrum of radio-frequency signals using unamplified fiber optic recirculation loops

An apparatus for generating a frequency spectrum of an RF signal comprising a gate switch for generating a series of pulses from a laser of wavelength lambda modulated by an input RF signal, a first fiber optical loop for circulating a first percentage of a first pulse of the series of pulses from the gate switch, for a predetermined number of cycles n where each cycle takes time t1, a second fiber optical loop for conducting a second percentage of the first pulse for predetermined number of cycles “k”, where each cycle takes time t2, where t2*k=t1*n, a first switch with a first state for coupling the first pulse from the gate switch to a coupler, the coupler coupling the first pulse into the first fiber optical loop and tapping replicas of the pulse from the first fiber optical loop, and a second state for coupling the second percentage of the first pulse to the coupler to increase intensity of the tapped replica pulses, a processor for correlating the replicas of the pulse with each other to produce a set of data points comprising a plurality of multiplexed correlated pulses and transforming the data points into a channelized frequency spectrum of the input RF signal.

FIELD OF INVENTION

Embodiments of the present invention generally relate to radio-frequency receiving system and, more particularly, to a method and apparatus for analyzing the spectrum of radio-frequency signals using unamplified fiber optic recirculation loops.

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 and military RF systems are transmitted across multiple frequencies (e.g., using frequency hopping and/or spread spectrum techniques) in a short time window. 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 in an optical signal processing section of the receiver. Such techniques, unfortunately, are prone to resonant noise and system instability due to the use of amplifiers within the optical signal processing section.

Therefore, there is a need in the art for an improved 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 a method and an apparatus for generating a frequency spectrum of an RF signal comprising a gate switch for generating a series of pulses from a laser of wavelength lambda (λ) modulated by an input RF signal, a first fiber optical loop for circulating a first percentage of a first pulse of the series of pulses from the gate switch, for a predetermined number of cycles n where each cycle takes time t1, a second fiber optical buffer delay loop for conducting a second percentage of the first pulse for a predetermined number of cycles “k”, where each cycle takes time t2, where t2*k=t1*n, a first switch with a first state for coupling the first pulse from the gate switch to a coupler, the coupler coupling the first pulse into the first fiber optical loop and tapping replicas of the pulse from the first fiber optical loop, and a second state for coupling the second percentage of the first pulse to the coupler to increase intensity of the tapped replica pulses, a processor for correlating the replicas of the pulse with each other to produce a set of data points comprising a plurality of multiplexed correlated pulses and transforming the data points into a channelized frequency spectrum of the input RF signal.

Embodiments of the present invention further relate to a method and an apparatus for generating a frequency spectrum of an RF signal comprising a gate switch for generating a series of pulses from a laser of wavelength lambda modulated by an input RF signal, fiber optical loop for circulating a first pulse of the series of pulses from the gate switch, for a predetermined number of cycles n where each cycle takes time t1, a first switch, coupled to the gate switch, in a first state for coupling the first pulse from the gate switch to a coupler, the coupler coupling a first percentage of the first pulse into the fiber optical loop and tapping a second percentage of the first pulse out of the first fiber optical loop as a replica pulse, and a second state for coupling the fiber optical loop to the coupler, and a processor for correlating a predetermined amount of replicas of the pulse with each other to produce a set of data points comprising a plurality of multiplexed correlated pulses and transforming the data points into a channelized frequency spectrum of the input RF signal.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention comprise an apparatus for generating a large number of re-circulated replica signal pulses in the time domain from a single input optical pulse that carry a RF signal with two side bands while avoiding noise by use of an unamplified, extreme low-loss, fiber-optic recirculation loop circuit. In some embodiments fiber-optic recirculation loop circuit comprises two fiber optical loops, where one fiber optical loop recycles and introduces a buffer delay in the original input pulse to augment the intensity of the pulse signal in the other fiber optical loop. Each replica pulse carries two copies of the RF signal, the copies having a relative time delay due to the chromatic dispersion of the fiber optical loop. The replica signal pulses are then coupled to a photodetector to obtain the RF signal that is expanded in time as a series of delayed pulses that may be later correlated. A processor is configured to perform a Fast-Fourier Transform (FFT), using a transform module, on the correlated pulses, generating a channelized frequency spectrum.

When searching for unknown RF signals, at ten of hundreds of gigahertz, thousands, or possibly, millions of transmitter signal may be received by a wide bandwidth receiver, making it difficult to determine which signal may be a signal of interest. In such a situation, higher frequency resolution is desired in order to search the carrier frequency of the signal of interest and then, perform intermediate frequency (IF) down-converting. Accordingly, a low-loss fiber optic recirculation circuit can be used to perform such an analysis using a significant amount of generated pulses, as described inFIG. 1. However, to minimize processing time, it is desirable to perform a two step searching process by performing an, initial low-resolution “quick” search by using a single fiber optical loop circuit as shown inFIG. 2that provides only a small amount of generated pulses for analysis. Once a signal of interest is found, the high resolution processor shown inFIG. 1is used to narrow down the frequency channel as described below.

FIG. 1is a block diagram of an apparatus100for detecting radio-frequency (RF) signals using fiber optic recirculation loops in accordance with exemplary embodiments of the present invention. The apparatus100comprises a laser102, an optical modulator104, a gate switch106, a 1×2 switch108, a 2×2 coupler110, a fiber optic recirculation loop112, a second 1×2 switch114, an amplifier116, a 2×2 switch118, a second fiber optical recirculation loop120, a control circuit121, a photo-detector117and a computer154. The computer154comprises a processor159and memory157. The memory157comprises a transform module156and stores data points160.

According to an exemplary embodiment of the present invention, the apparatus100processes an RF input signal101and produces frequency spectrum158representing information carried by the RF input signal101. The input signal101is an RF signal and the laser102is a carrier wave light signal.

The RF input signal101may 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 laser102provides laser light of wavelength lambda (λ). The optical modulator104modulates light from the laser102with the RF input signal101, creating an RF modulated light wave105with two sidebands around the laser carrier in the frequency domain. According to an exemplary embodiment, the laser102is a continuous wave laser (CWL). In other embodiments, two lasers with varying wavelengths are generated and modulated by the modulator104. Also according to an exemplary embodiment of the present invention, the optical modulator104is a Mach Zehnder modulator, though those of ordinary skill in the art will recognize that any equivalent modular may be used. The gate switch106(e.g., an optical gate switch) converts the modulated light into an RF modulated optical pulse. After an initial output pulse107of the modulated light wave105leaves the gate switch106, the gate switch106closes so that for a predetermined length of time further pulses cannot enter the loop112to cause distortion and noise. Thereafter, the control circuit121controls opening and closing of the gate switch106so as to create a series of pulses which enter the loop112in a periodic fashion.

According to an exemplary embodiment, the optical gate switch106is a fast and high extinction ratio optical gate switch, and generates short pulse107from the modulated laser light105at an output port G1. According to some embodiments, the optical power of the pulse generated at port G1has a range of 10 mW to 100 mW. The 1×2 switch108couples the output pulse from output port G1to input port C1of the 2×2 coupler110. The gate switch106also has an output port G2, which may be used to analyze the RF signal by another circuit (not shown), unrelated to the apparatus100. The coupler110is used to couple the output pulse107into and out of the fiber recirculation loop112. According to an exemplary embodiment, the length of loop112is approximately 1.3 Km if regular telecom fiber is used and the total RF bandwidth is about 50 GHz.

Initially, the 1×2 switch108is set to a first state by the control circuit121when the gate switch106generates the pulse107, where the input A1is coupled to the output of the switch108. The 1×2 switch108has a second state, where the input A2is coupled to the output of switch108. After the input signal pulse107passes though switch108, the switch108will be switched to the second state and remains in that the second state for one full cycle of operation. Accordingly, when 1×2 switch108is set to the first state, the initial pulse107is coupled by switch108into port C1of coupler110. The coupler110is a weak 2×2 optical coupler, by way of non-limiting example, the coupler110has a 5:95 ratio. For example, 5% of the signal is coupled into the loop112from output port C4, while 95% of the signal is coupled from output port C3to the 1×2 switch114.

According to exemplary embodiments, the coupler110comprises two adjacent wave guides (not shown), for example, a waveguide from input port C1to output port C3and a waveguide from input port C2to output port C4. If a pulse is coupled with either one of the input ports, 95% of the pulse signal remains in the intended waveguide, while 5% is coupled off to the adjacent waveguide.

The first pulse107enters the coupler at input port C1. A first percentage (for example, 5%) of the power of the first pulse re-circulates in the loop112, through output port C4, a predetermined number of times (“n”), while a second percentage (for example, 95%) of the pulse107is output at port C3. According to some embodiments, the pulse circulates in the loop112fifty times, i.e., n=50. According to other embodiments, n=35. Those of ordinary skill in the art will recognize that n is a variable parameter based on the length L1of the loop112, to accommodate for various frequency bandwidths. After the initial pulse, each time the pulse circulates the loop once and arrives at port C2of the coupler110, the pulse signal is split once again so that a second percentage (for example, 95%) of the signal goes back into the loop112(i.e., the waveguide from port C2to C4carries the majority of the pulse signal) and a first percentage (for example 5%) exits the loop at port C3. After the initial pulse circulates the loop112once, the control circuit121sets the switch114to a second state, where the switch114outputs only at port B1, so the 1×2 switch114taps out a replica pulse (or, alternatively, a pulse pair with sidebands, where two initial lasers are used instead of laser102). If a pulse takes time VI to travel once fully through loop112, where the loop length of loop112is L1, a series of “n” pulse replicas will be generated at port C3, each pulse separated by a time t1from the subsequent pulse.

According to an exemplary embodiment, the loop112has a loss of approximately 0.17 db per kilometer and the length of loop112is about 1.3 km. Further, 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 loop112may be a dispersion shifted fiber where there is non-dispersion around the operating wavelength of the light generated by laser102.

Initially the 1×2 switch114is in a first state, where the input of switch114is coupled to output port B2of switch114, so that the input from port C3is coupled directly to the output B2. After the initial pulse is output at output B2of switch114, the control circuit121sets switch108to a second state, where the input of switch114is coupled to the output port B1of switch114.

The initial pulse of the series of pulses is coupled from output port B2of switch114to an input port D1of the 2×2 switch118. When the initial pulse is coupled to switch118, the control circuit121controls the switch118so that the initial pulse at input port D1is coupled so as to recirculate in the loop120through output port D4. After the initial pulse enters the loop120, the control circuit121sets the switch118to the “D2-D4” setting where the input port D2will couple the pulse to the output port D4, causing the pulse to travel into the loop120, for a predetermined number of times (“k”). According to exemplary embodiments of the present invention, the loop120is significantly larger than loop112. In some embodiments, the loop120is ten times the length of loop112. The pulse travels through loop120to complete a single cycle in a predetermined duration of time t2. Accordingly, time k*t2is the duration of time the pulse remains in loop120, as well as the duration of time taken by the initial pulse to travel through loop112for n cycles.

After the pulse travels the loop120for n cycles in time k*t2, the control circuit121controls port D3of the 2×2 switch118to be coupled to port A2of switch108, in order to allow the pulse to reenter the loop112via the coupler110. By way of a non-limiting example, if n=35, five percent of the initial pulse will travel through the loop112thirty-five times (cycles) while 95% of the initial pulse travels through loop120“k” times. However, due to loss in the loop112, as well as the successive tapping out of a portion of the pulse signal, the pulse signal will lose energy during each cycle. When the subsequent pulse exits the switch108and enters the loop112, 95% of the initial pulse from loop120is coupled with the subsequent pulse via switch108and input C1, the signal intensity is increased, and so on for the next 35 pulses.

Optionally, this embodiment can be simplified by making k=1 and removing the 2×2 switch118. Therefore the total length of the fiber loop120is n times the length of the first loop112. According to another embodiment, an optional optical amplifier140may be added in the buffer delay loop120to compensate for fiber loss so that the recycled subsequent first pulse sent to C1has the same amplitude of the original first input pulse. According to yet another embodiment, an optional switch141and an optical filter142may be added after the optional amplifier140as a time domain and frequency domain noise filter to filter out the noise generated from the amplifier140.

After the initial pulse enters switch114and outputs at port B2, the control circuit121switches the output of switch114to output port B1. This allows 5% of the pulse signal in the loop112to be tapped out of the loop to generate a replica pulse at output port B1. The series of n pulses are then coupled from the output port B1of switch114to the optical amplifier116. According to an exemplary embodiment of the present invention, the amplifier116is an Erbium doped fiber amplifier (EDFA), though the present invention is not limited the type of amplifier used. By placing the amplifier116outside of the loop112, resonant noise in the loop112is reduced significantly, increasing the signal to noise ratio.

According to exemplary embodiments of the present invention, all fiber connections to couplers and switches in the circuit shown inFIG. 1are spliced connections. According to some embodiments, the values n and k may be increased to reduce the total amount of instances when the pulse passes through switch108, switch114and switch118to avoid signal loss. According to some instances, the switch118may be removed entirely and the loop120is composed instead of a long fiber that is n×L1in length.

The process of recirculating a pulse in the loop112n times may be repeated until a desired number of replica pulses are obtained. The amplifier116couples the set of replica pulses to a photodetector117to obtain the original RF signal expanded in time as a series of pulses delayed by time t1.

The output RF electronic pulse signals from the photodetector117is coupled to a data acquisition and processing computer154. The detail processing and mathematics is described in commonly assigned U.S. application Ser. No. 13/371,556 filed on Feb. 13, 2012, herein incorporated by reference in its entirety.

The RF modulated optical pulse has two side bands around the frequency carrier and therefore contains two copies of the RF modulated pulse. When the pulse enters the fiber optical loop112, and performs one loop cycle with travel time t1, the two RF side band pulses will travel at a slight relative time delay δt1between each other due to the chromatic dispersion of the loop112. According to embodiments of the present invention, N replica pulse pairs are generated by this process. The relative time delay generated between each pulse pairs are, accordingly, δt1, 2×δt1, 3δt1. . . N×δt1. These pulse replica pairs provide the data points for time-domain autocorrelation performed by the RF analog signal processor149.

The computer154, performs an analog to digital conversion on the auto-correlates the replica pulses signal produced by the processor149, and generates a set of data points160which are then stored in memory157.

The computer154also performs data normalization for calibrating on a set of calibration signal with a known RF frequency to produce a weight function for the periodic amplitude changes due to the recirculation loop circuit without RF. In the digital domain, the computer154may further normalize the data set from the real signal using that weight function and then uses the transform module156to apply a Fourier Transform to transform the normalized auto-correlated data from the time domain to the frequency domain, thereby obtaining the RF spectrum of the input RF pulse101. In an exemplary embodiment, the memory157may 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 Fast Fourier Transformed data points160generate a channelized frequency spectrum which is obtained using signal processing techniques known to those of ordinary skill in the art.

FIG. 2is a block diagram of an apparatus200for detecting radio-frequency signals using a fiber optic recirculation loop in accordance with another exemplary embodiment of the present invention. In some instances, it is useful to generate a limited number of recirculation pulses; for example, several hundred pulses can be used to quickly provide a low-resolution frequency spectrum and update it rapidly.FIG. 2depicts apparatus200for use in those instances, where a laser modulated by an input RF signal travels through the fiber-recirculation loop via a low-loss, high speed optical switch to generate replica pulses. The replica pulses are then amplified and an RF signal in the time domain is extracted from the replica pulses. The RF signal in the time domain is auto-correlated to generate a set of data points and a transformation is applied to the set of data points to generate a frequency spectrum of the RF input signal101.

According to an exemplary embodiment of the present invention, the apparatus200processes the RF input signal101and produces frequency spectrum258representing information carried by the input signal101. The input signal101is an RF signal and the laser202is a carrier wave light signal.

The RF input signal101may 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 laser202provide laser light of wavelength λ. The optical modulator204modulates light from the laser202with the RF input signal101, creating an RF modulated light wave205with two sidebands around the laser carrier in the frequency domain. According to an exemplary embodiment, the laser202generates a continuous wave laser. Also according to an exemplary embodiment of the present invention, the optical modulator204is a Mach Zehnder modulator. According to yet another embodiment, two lasers with differing wavelengths may be used in place of laser102. The gate switch206(e.g., an optical gate switch) converts the modulated light into a pair of RF modulated optical pulse. Once a pulse enters the gate switch206, the control circuit221causes the gate switch206to close so further pulses cannot enter the loop209to cause distortion and noise.

The optical pulse generated by the gate switch206is coupled to the input port A1of the 2×1 switch208. According to exemplary embodiments of the present invention, the switch208is a low-loss, high-speed fiber switch. Initially, the switch208is set to a first state where the input from port A1is coupled to the output port of switch208, thereby coupling the output of switch208to the input of a 1×2 coupler210. According to an exemplary embodiment, the coupler210is a weak 1×2 optical splitter with a large splitting ratio, for example 1% to 99%, though other weak ratio couplers may also be substituted. Accordingly, if a 1:99 coupler is used, 99% of the initial pulse is output at port C1of coupler210into the loop209, where the loop209has a length L. Once the initial optical pulse enters loop209, the control circuit221sets the switch208to a second state, where input port A2of switch208is coupled to the output port of switch208, to close the loop209allowing the pulse to travel through the loop209for a duration of time.

Each time the initial optical pulse cycles through the loop209, the coupler210outputs a small portion for example, 1%, of the optical pulse signal to generate a replica pulse, thereby generating a series pulse train at C2. The pulse train is coupled to the input of an amplifier212to amplify the signal. The amplified signal is coupled to a photo-detector117to obtain an original RF signal expanded in time as a series of delayed pulses.

The output RF electronic pulse signals from the photodetector117is coupled to the RF signal processor149and produces the auto-correlation data that is collected by the computer154. The detail processing and mathematics is described in commonly assigned U.S. application Ser. No. 13/371,556 filed on Feb. 13, 2012, herein incorporated by reference in its entirety.

The RF modulated optical pulse has two side bands around the frequency carrier and therefore contains two copies of the RF modulated pulse. When the pulse enters the fiber optical loop112, and performs one loop cycle with travel time t1, the two RF side band pulses will travel at a slight relative time delay δt1between each other due to the chromatic dispersion of the loop112. According to embodiments of the present invention, N replica pulse pairs are generated by this process. The relative time delay generated between each pulse pairs are, accordingly, δt1, 2×δt1, 3δt1. . . N×δt1. These pulse replica pairs provide the data points for time-domain autocorrelation performed by the RF signal processor149.

The computer154performs an analog to digital conversion on auto-correlation signal from processor149, and generates a set of data points160which are then stored in memory157.

The computer154also performs data normalization for calibrating on a set of calibration signal with a known RF frequency to produce a weight function for the periodic amplitude changes due to the recirculation loop circuit without RF. In the digital domain, the computer154may further normalize the data set from the real signal using that weight function and then use the transform module156to apply a Fourier Transform to transform the normalized auto-correlated data from the time domain to the frequency domain, thereby obtaining the RF spectrum of the input RF pulse101. In, an exemplary embodiment, the memory157may 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.

According to the embodiments shown inFIGS. 1 and 2, the relative intensity difference between successive pulse replicas is predetermined by the loop circuits loss, including the loss, due to signal splitting and tapping out. To obtain normalized pulse replica data sets, the loss or intensity fluctuation may be experimentally measured by recording the pulse intensity with an un-modulated laser input pulse or a laser input pulse modulated by a single known RF frequency to perform the normalization in the digital domain.

According to an exemplary embodiment, apparatus200can be used in conjunction with apparatus100. In order to perform a “quick” search of a spectrum with low resolution, apparatus200is used. Subsequently, the apparatus100is used for higher resolution frequency spectrum that performs a “zoom in” function to resolve the signal of interest.

FIG. 3is a flow diagram of method300for detecting radio-frequency signals using an optical loop in accordance with exemplary embodiments of the present invention. The method begins at step302and proceeds to step304, where a first percentage of a pulse of one or more modulated laser light beams are conducted in a first fiber optical loop by a coupler, such as coupler110. At step306, the coupler taps out a second percentage of the pulse from the first fiber optical loop. At step308, the second percentage of the pulse is conducted in a second fiber optical loop.

At step310, the method300determines whether the pulse has travelled n cycles in the first fiber optical loop, and if not, the method returns, to step304. If n cycles have been completed, the method300proceeds to step312. At step312, a switch is modified to a second state, where a second percentage of the pulse is coupled from the second fiber optical loop back into the first fiber optical loop to augment the intensity of the signal.

At step314, the tapped out replica pulses are autocorrelated to generate a set of data points. At step316, the method300determines whether a predetermined number of data points have been stored. If a predetermined number (X) of data points have not been stored, the method proceeds to step304where more replica pulses are tapped out of the first fiber optical loop. If a predetermined number of data points have been stored, the method proceeds to step318.

At step318the computer154performs a data normalization and FFT on the X points to generate a frequency spectrum at step318. At step320, the method terminates, when all pulses have been stored and a frequency spectrum has been generated for the input RF signal.

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.