Pulse rate multiplier

Abstract

A pulse-rate multiplication system (20, 54) includes a pulse generator (24) and a delay line (32). The pulse generator is configured to generate a sequence of input pulses having a pulse duration and a Pulse Repetition Interval (PRI). The delay line includes an array of equally spaced reflectors (36), adjacent ones of the reflectors having a roundtrip time (RTT) therebetween. The delay line is configured to output a reflected signal including a sequence of output pulses whose rate is multiplied relative to the sequence of input pulses. M denotes a ratio between the RTT and the pulse duration, N denotes a number of the reflectors in the array, and M and N are set to be co-prime.

Description

[0001] PULSE RATE MULTIPLIER

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Patent Application 63 / 671,303, filed July 15, 2024, whose disclosure is incorporated herein by reference.

[0004] FIELD OF THE INVENTION

[0005] The present invention relates generally to pulse rate multiplication, and more particularly to pulse rate multiplication using a delay line with an array of equally spaced reflectors.

[0006] BACKGROUND OF THE INVENTION

[0007] Pulse rate multiplication techniques are used in various fields of technology to increase the frequency of a pulse train. This process is particularly useful in applications where high- frequency pulse trains are required but may be difficult to generate directly. Example applications include data transmission, signal processing, medical systems, sensing systems and many others.

[0008] In many scenarios, the ability to generate high-frequency pulse trains is limited by the capabilities of available pulse generators or by system constraints. For instance, in optical systems, the maximum pulse rate may be restricted by the modulation speed of lasers or other light sources. Similarly, in electronic circuits, the upper limit of pulse generation might be determined by the switching speed of transistors or other components.

[0009] SUMMARY OF THE INVENTION

[0010] An embodiment of the present invention that is described herein provides a pulse-rate multiplication system including a pulse generator and a delay line. The pulse generator is configured to generate a sequence of input pulses having a pulse duration and a Pulse Repetition Interval (PRI). The delay line includes an array of equally spaced reflectors, adjacent ones of the reflectors having a roundtrip time (RTT) therebetween. The delay line is configured to output a reflected signal including a sequence of output pulses whose rate is multiplied relative to the sequence of input pulses. M denotes a ratio between the RTT and the pulse duration, N denotes a number of the reflectors in the array, and M and N are set to be coprime. In some embodiments, a ratio between the PRI and the pulse duration is equal to N.

[0011] In an embodiment, the pulse generator includes a laser, the delay line includes an optical fiber, and the reflectors include Fiber Bragg Gratings (FBGs). In another embodiment, the pulse generator includes a laser, the delay line includes an optical fiber, and the reflectors include optical couplers terminated with respective mirrors. In yet another embodiment, the pulse generator includes a laser, the delay line includes an input optical fiber and an output optical fiber, and the reflectors include optical couplers that couple the input pulses from the input optical fiber to the output optical fiber.

[0012] In a disclosed embodiment, the PRI and the RTT are integer multiples of the pulse duration. In an embodiment, in steady state, the reflected signal has a duty cycle of unity. In some embodiments, the system further includes (i) a spread-spectrum modulator configured to modulate the input pulses with a spread-spectrum code and to apply the modulated pulses to the delay line, and (ii) a correlator configured to correlate the reflected signal with the spreadspectrum code so as to reproduce the output pulses.

[0013] There is additionally provided, in accordance with an embodiment that is described herein, a method for pulse-rate multiplication. The method includes generating a sequence of input pulses having a pulse duration and a Pulse Repetition Interval (PRI). The input pulses are applied to a delay line including an array of equally spaced reflectors, adjacent ones of the reflectors having a roundtrip time (RTT) therebetween, so as to output a reflected signal including a sequence of output pulses whose rate is multiplied relative to the sequence of input pulses. M denotes a ratio between the RTT and the pulse duration, N denotes a number of the reflectors in the array, and M and N are set to be co-prime.

[0014] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

[0015] BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figs. 1 and 2 are block diagrams that schematically illustrate Distributed Acoustic Sensing (DAS) systems, in accordance with embodiments of the present invention;

[0017] Fig. 3 is a diagram that schematically illustrates a delay line comprising an optical fiber with equally spaced Fiber Bragg Gratings (FBGs), in accordance with an embodiment of the present invention;

[0018] Fig. 4 is a diagram that schematically illustrates a delay line comprising a pair of optical fibers with equally spaced couplers, in accordance with an embodiment of the present invention;

[0019] Figs. 5-10 are diagrams that schematically illustrate examples of pulse rate multiplication using a delay line having equally spaced reflectors, in accordance with embodiments of the present invention; and Fig. 11 is a flow chart that schematically illustrates a method for pulse rate multiplication, in accordance with an embodiment of the present invention.

[0020] DETAILED DESCRIPTION OF EMBODIMENTS

[0021] OVERVIEW

[0022] Embodiments of the present invention that are described herein address the challenge of generating high-frequency pulse trains in various technological applications where direct generation of such signals may be difficult or impractical. In many fields, including data transmission, signal processing, and distributed sensing systems, there is a growing demand for higher frequency signals. However, the ability to produce these high-frequency pulse trains is often limited by the capabilities of available pulse generators or by system constraints.

[0023] To overcome these challenges, disclosed embodiments provide a novel pulse rate multiplication system that employs a delay line with an array of equally spaced reflectors. For a pulse train having a given pulse width (pulse duration) and a given pulse rate (or Pulse Repetition Interval - PRI), the disclosed technique enables multiplying the pulse rate to the maximal possible rate, while keeping the pulse durations unchanged. This property is highly desirable, for example, in Distributed Acoustic Sensing (DAS) application.

[0024] In an example embodiment, the system comprises a pulse generator that produces a sequence of input pulses with a specific pulse duration and PRI. The input pulses are then fed into a delay line containing an array of equally spaced reflectors. The spacing between adjacent reflectors in the array corresponds to a certain roundtrip time (RTT). Each input pulse is reflected from the various reflectors of the array. The end result is a sequence of output pulses whose pulse rate is multiplied relative to that of the sequence of input pulses.

[0025] In some embodiments, the system is configured with two critical parameters set to be co-prime: (i) the ratio between the RTT and the pulse duration, and (ii) the number of reflectors in the array. This co-prime relationship achieves the maximal possible pulse rate multiplication factor permitted by the pulse durations (i.e., the maximal multiplication factor that still avoids overlap between output pulses). When the co-prime condition is fulfilled, the sequence of output pulses has a duty cycle of unity, meaning that the output pulses occupy the entire time domain without gaps. In some embodiments, in addition to the co-prime condition, the ratio between the PRI and the pulse duration is set to be an integer number that is equal to the number of reflectors in the array.

[0026] When using the disclosed technique, the same system can be used with various pulse durations, and achieve maximal pulse rate multiplication for any pulse duration. This degree of freedom is important for controlling the required system bandwidth. The above properties are advantageous in many applications. The embodiments described herein refer mainly to DAS by way of non-limiting example. DAS systems and techniques are described, for example, in U.S. Patent Application Publication 2022 / 0178723. A DAS system that uses arrays of reflectors is sometimes referred to as Quasi-DAS (Q-DAS).

[0027] In a DAS application, the input and output pulses are optical pulses, the delay line comprises an optical fiber or a pair of fibers, and the reflectors are optical reflectors. Generally, however, the disclosed techniques are not limited to optical applications and can be implemented, for example, with electrical input and output pulses, an electrical delay line, and reflectors that reflect the electrical input pulses.

[0028] EXAMPLE SYSTEM CONFIGURATIONS

[0029] Fig. 1 is a block diagram that schematically illustrates a Distributed Acoustic Sensing (DAS) system 20, in accordance with an embodiment of the present invention. Systems such as system 20 can be used in any suitable field or application that involves data collection from sensors. Several non-limiting examples include networks of perimeter intrusion detection sensors, transportation-related sensor systems such as along railways, roads, or tunnels, energy-related sensor networks such as in power stations, oil rigs, or gas fields, and / or geophone sensor networks used in seismology, to name just a few.

[0030] System 20 comprises a pulse generator 24, an optical fiber 32 and a detector 40. Fiber 32 comprises an array of equally spaced reflectors 36 embedded therein. Reflectors 36 are referred to as "weak reflectors" as they reflect only a small portion of each input pulse. A typical useful range of reflection coefficients is between 0.01% and 1%, although other values can also be used. Fiber 32 can be viewed as an example of a delay line having an array of equally spaced reflectors.

[0031] Pulse generator 24 generates a sequence of input pulses and transmits them into fiber 32 via a circulator 28. Each input pulse traverses fiber 32 and is reflected from the various reflectors 36. At each reflector 36, a small portion of the input pulse is reflected back toward circulator 28, forming a pulse referred to as an output pulse. The remaining portion of the input pulse continues to traverse fiber 32 to the next reflector 36. The superposition of the reflections of the various input pulses from the various reflectors forms a sequence of output pulses that exits fiber 32.

[0032] Detector 40 receives the sequence of output pulses from fiber 32 via circulator 28. Detector 40 converts the sequence of optical output pulses into an electrical signal for further processing and analysis. In a DAS application, for example, the purpose of the analysis may be to detect a change in the physical properties of fiber 32, occurring at a specific location along the fiber. Such a change may be indicative of an event of interest occurring at that location.

[0033] In the present example, pulse generator 24 comprises a laser 44, a function generator 48 and a modulator 52. Laser 44 serves as a source of coherent light, generating a Continuous- Wave (CW) optical signal. Function generator 48 produces electrical signals that control the modulation of the laser output. Modulator 52 receives inputs from both laser 44 and function generator 48, and modulates the continuous laser light into discrete pulses with specific characteristics such as pulse duration and Pulse Repetition Interval (PRI).

[0034] Fig. 2 is a block diagram that schematically illustrates a DAS system 54, in accordance with an alternative embodiment of the present invention. Unlike system 20 of Fig. 1, system 54 of Fig. 2 performs coherent detection of the output pulses.

[0035] In system 54, a portion of the CW optical signal generated by laser 44 is sampled by an optical splitter 56, to serve as a reference optical signal in the coherent detection. System 54 further comprises a 90-degree optical hybrid 60. Hybrid 60 receives an input (i) the reference optical signal from laser 44 and (ii) the sequence of optical output pulses from fiber 32 (via circulator 28). Hybrid 66 outputs two Quadrature baseband signal components, which are detected by a pair of balanced detectors 64.

[0036] The configurations of systems 20 and 54, and their components, as described hereinabove and illustrated in Figs. 1 and 2, are example configurations that are shown purely for the sake of conceptual clarity. Any other suitable configurations can be used in alternative embodiments. The different elements of the disclosed systems, and their components, may be implemented using suitable hardware, such as in using discrete optical and / or electronic components, one or more Integrated Circuits (ICs) or Field-Programmable Gate Arrays (FPGAs), using software, using software, or using a combination of hardware and software elements.

[0037] EXAMPLE FIBER DELAY LINE CONFIGURATIONS

[0038] Fig. 3 is a diagram that schematically illustrates a delay line comprising a single optical fiber 68, in accordance with an embodiment of the present invention. This delay line can be used, for example, in system 20 of Fig. 1 or in system 54 of Fig. 2. In the configuration of Fig. 3, an array of multiple equally spaced Fiber Bragg Gratings (FBGs) 72 is embedded in the core of fiber 68. FBGs 72 serve as weak reflectors. Fig. 4 is a diagram that schematically illustrates a delay line comprising a pair of optical fibers 76 and 80, in accordance with an alternative embodiment of the present invention. Fiber 76 is referred to as an input fiber, and is used for propagation of the input pulses. Fiber 80 is referred to as an output fiber, and is used for propagation of the output pulses. This delay line, too, can be used in system 20 of Fig. 1 or in system 54 of Fig. 2. When using this configuration, circulator 28 is omitted.

[0039] In the configuration of Fig. 4, the weak reflectors are implemented using optical couplers 84 that are inserted along the pair of fibers. When an input pulse reaches a given coupler 84 in input fiber 76, the coupler couples a small portion of the input pulse to output fiber 80, so as to form an output pulse that propagates backwards along fiber 80. The remaining portion of the input pulse continues to propagate along input fiber 76. The directions of propagation are marked by arrows in the figure.

[0040] In one example implementation, the number of reflectors (FBGs 72 or couplers 84) is 100 and the spacing between adjacent reflectors is 100m. Alternatively, any other suitable values can be used.

[0041] The delay line configurations shown in Figs. 3 and 4 are example configurations that are chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable delay line configuration can be used. The physical medium of the delay line, for example, is not limited to an optical fiber, and may alternatively comprise an optical waveguide, air, an electrical transmission line or any other suitable medium. The array of reflectors may comprise any suitable element or structure that reflects a portion of the input pulses.

[0042] In an optical delay line, for example, the reflectors may alternatively comprise Mach- Zehnder (MZ) devices. Alternatively, the reflectors can be implemented using transmissive elements rather than reflective elements, e.g., using couplers. In one configuration of this sort, each reflector comprises an optical coupler that is terminated with a mirror, so that a small portion of the light propagating in the fiber is reflected back in the opposite direction.

[0043] CHOICE OF PARAMETERS FOR MAXIMAL PULSE-RATE MULTIPLICATION

[0044] Consider a sequence of input pulses to be rate-multiplied. All the input pulses in the sequence are assumed to have the same wavelength. The reflectors (e.g., FBGs) are assumed to be nominally similar with the same reflection spectrum. Maximal pulse-rate multiplication can be achieved by proper choice of the pulse duration denoted T, the scan period (also referred to as PRI) denoted Ts, the roundtrip time (RTT) between consecutive reflectors denoted Trt, and the number of reflectors in the array denoted N.

[0045] In the present context, the term "pulse duration" refers to the time frame that confines the energy of the pulse, such that any leading or trailing parts of the pulse are negligible. We may also add some empty margins to the pulse to improve the tolerance to position errors of the reflectors and jitter.

[0046] In some embodiments, both Tsand Trtare set to be integer multiples of T. This setting creates, for the reflected output pulses, a time axis of equally spaced time slots of size T. Each such time slot may be empty or may be populated with one or more reflected pulses. To achieve maximal pulse-rate multiplication, the interrogation parameters (Tsand T) should be set so that the sequence of output pulses, in steady state, fills the entire scan period (i.e., populates all the time slots).

[0047] For simplicity, a sequence of output pulses that fills all the time slots in steady state is referred to herein as having a duty cycle of unity (1). Accordingly, we design the interrogation parameters so that TS / T = N and Trt / i = M. N denotes the number of reflectors in the array. M (the ratio between (i) the roundtrip time between adjacent reflectors and (ii) the pulse duration) is an integer number. Assuming steady state, i.e., that the interrogation started at t = — oo, the normalized delays of the output pulses can be expressed as

[0048] In the expression above D(i,j) is a oo x N matrix of integers. Our goal is to find the relation between N and M that will guarantee that D(i,j) will contain all integers without repetitions. It can be shown that this condition is met when N and M are co-prime integers (i.e., integers that have no common factor, or, differently put, integers for which the only positive integer that is a divisor of both of them is 1). With this choice of N and M, the sequence of output pulses has a duty cycle of 1 with no overlaps (i.e., has exactly one output pulse in each time slot). A formal proof can be found in U.S. Provisional Patent Application 63 / 671,303, cited above. PULSE-RATE MULTIPLICATION WALK-THROUGH AND EXAMPLES

[0049] Figs. 5-10 are diagrams that schematically illustrate examples of pulse rate multiplication using a delay line having equally spaced reflectors, in accordance with embodiments of the present invention.

[0050] Figs. 5-7 demonstrate the general operation of a delay line with an array of equally spaced weak reflectors. Figs. 8-10 show examples of maximal pulse-rate multiplication with proper choice of interrogation parameters (A and M co-prime). In all six figures, the left-hand side illustrates the input pulses, and the right-hand side illustrates the output signals. The function H(t) denotes the response of the delay line.

[0051] Fig. 5 illustrates the case of a single input pulse. As seen, the single input pulse is reflected from the N reflectors in the array, thereby generating a sequence of N output pulses spaced apart by Trt.

[0052] Figs. 6 and 7 show the steady-state response of the delay line to a sequence of input pulses, for N and M that are chosen arbitrarily. In the example of Fig. 6, the sequence of output pulses has a duty cycle <1, without overlap. In other words, output pulses do not overlap one another, but certain gaps exist between them. In the example of Fig. 7, the sequence of output pulses has a duty cycle <1, with overlap. As seen in the figure, some of the time slots contain more than one output pulse. In addition, some gaps exist between output pulses.

[0053] Fig. 8 shows an example of maximal pulse-rate multiplication with N = 8 and M = 3. As seen on the right-hand side of the figure, the output pulses occupy all time slots (i.e., duty cycle of 1) without overlap (i.e., only a single output pulse per time slot). The number within each output pulse denotes the index of the reflector that reflected the output pulse.

[0054] Fig. 9 shows another example of maximal pulse-rate multiplication with co-prime N and M, in this case N = 8 and M = 5. Fig. 10 shows yet another example of co-prime N and M, namely N = 8 and M = 13.

[0055] GENERAL METHOD DESCRIPTION

[0056] Fig. 11 is a flow chart that schematically illustrates a method for pulse rate multiplication, in accordance with an embodiment of the present invention. The method begins with pulse generator 24 generating a sequence of input pulses, at an input sequence generation stage 90. At a pulse-rate multiplication stage 94, delay line 32 receives the sequence of input pulses and, in response, outputs a sequence of output pulses whose pulse rate is multiplied relative to that of the sequence of input pulses. At an output stage 98, detector 40 or detectors 64 receive the sequence of output pulses from delay line 32.

[0057] ADDITIONAL EMBODIMENTS AND VARIATIONS

[0058] The disclosed techniques can be used in any suitable application that requires rate multiplication of a sequence of pulses, not necessarily for sensing purposes. One example application is pulse-rate multiplication for pulses generated by a frequency-hopping laser. Such a laser typically has a finite and non-negligible frequency-hopping time. The disclosed technique enables generating a sequence of optical pulses that hop in frequency much more rapidly than the capability of the laser. To this end, the pulses generated by the laser can be applied to any of the disclosed delay lines (e.g., fiber with array of weak reflectors). In this particular application, the reflectors should be sufficiently broadband to cover the full bandwidth over which the laser frequency hops. Assuming the system parameters are set as explained above, the reflected signal at the output of the delay line will comprise a frequency hopping signal with a duty cycle of 1.

[0059] In some embodiments, the system further comprises (i) a spread-spectrum modulator that modulates the pulses with a spread-spectrum code and applies the modulated pulses to the delay line, and (ii) a correlator that correlates the reflected signal with the spread-spectrum code so as to reconstruct the rate-multiplied pulses. In these embodiments, the signal transmitted into the delay line comprises a sequence of spread-spectrum codes that are generated from the short pulses. The waveforms that traverse the delay line and are reflected by the reflectors are thus not necessarily short, and typically overlap with one another. The correlator at the output of the delay line compresses the waveforms to reproduce the rate- multiplied pulses. In the present context, the definitions of pulse duration, PRI and roundtrip time, the ratios between M and N, and the notion of duty-cycle, refer to the original short pulses.

[0060] It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.

Claims

CLAIMS1. A pulse-rate multiplication system, comprising: a pulse generator configured to generate a sequence of input pulses having a pulse duration and a Pulse Repetition Interval (PRI); and a delay line comprising an array of equally spaced reflectors, adjacent ones of the reflectors having a roundtrip time (RTT) therebetween, the delay line configured to output a reflected signal comprising a sequence of output pulses whose rate is multiplied relative to the sequence of input pulses, wherein M denotes a ratio between the RTT and the pulse duration, wherein N denotes a number of the reflectors in the array, and wherein M and N are set to be co-prime.

2. The system according to claim 1, wherein a ratio between the PRI and the pulse duration is equal to N.

3. The system according to claim 1, wherein the pulse generator comprises a laser, the delay line comprises an optical fiber, and the reflectors comprise Fiber Bragg Gratings (FBGs).

4. The system according to claim 1, wherein the pulse generator comprises a laser, the delay line comprises an optical fiber, and the reflectors comprise optical couplers terminated with respective mirrors.

5. The system according to claim 1, wherein the pulse generator comprises a laser, the delay line comprises an input optical fiber and an output optical fiber, and the reflectors comprise optical couplers that couple the input pulses from the input optical fiber to the output optical fiber.

6. The system according to any of claims 1-5, wherein the PRI and the RTT are integer multiples of the pulse duration.

7. The system according to any of claims 1-5, wherein, in steady state, the reflected signal has a duty cycle of unity.

8. The system according to any of claims 1-5, further comprising: a spread-spectrum modulator configured to modulate the input pulses with a spreadspectrum code and to apply the modulated pulses to the delay line; and a correlator configured to correlate the reflected signal with the spread-spectrum code so as to reproduce the output pulses.

9. A method for pulse-rate multiplication, comprising: generating a sequence of input pulses having a pulse duration and a Pulse Repetition Interval (PRI); and applying the input pulses to a delay line comprising an array of equally spaced reflectors, adjacent ones of the reflectors having a roundtrip time (RTT) therebetween, so as to output a reflected signal comprising a sequence of output pulses whose rate is multiplied relative to the sequence of input pulses, wherein M denotes a ratio between the RTT and the pulse duration, wherein N denotes a number of the reflectors in the array, and wherein M and N are set to be co-prime.

10. The method according to claim 9, wherein a ratio between the PRI and the pulse duration is equal to N.

11. The method according to claim 9, wherein the input and output pulses comprise optical pulses, the delay line comprises an optical fiber, and the reflectors comprise Fiber Bragg Gratings (FBGs).

12. The method according to claim 9, wherein the input and output pulses comprise optical pulses, the delay line comprises an optical fiber, and the reflectors comprise optical couplers terminated with respective mirrors.

13. The method according to claim 9, wherein the input and output pulses comprise optical pulses, the delay line comprises an input optical fiber and an output optical fiber, and the reflectors comprise optical couplers that couple the input pulses from the input optical fiber to the output optical fiber.

14. The method according to any of claims 9-13, wherein the PRI and the RTT are integer multiples of the pulse duration.

15. The method according to any of claims 9-13, wherein, in steady state, the reflected signal has a duty cycle of unity.

16. The method according to any of claims 9-13, further comprising: modulating the input pulses with a spread-spectrum code and before applying the modulated pulses to the delay line; and correlating the reflected signal with the spread- spectrum code so as to reproduce the output pulses.

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