A system and method for identifying and / or characterising radio frequency signals

Abstract

A speckle pattern-based radio frequency (RF) spectrum analyser system the configured to repeatedly recalibrate during monitoring of a RF signal to compensate for changes in the speckle patterns over time resulting from changes in environmental factors, such as temperature and vibration, that would otherwise lead to disassociation between the measurements speckle pattern images. By recalibrating frequently, many of the stabilising features of the prior art systems can be omitted, including those used to minimise vibration of the whole or parts of the system which allows for significant reduction in the overall size of the apparatus.

Description

[0001] A System and Method for Identifying and / or Characterising Radio Frequency

[0002] Signals

[0003] The present invention relates to a system and method for identifying and / or characterising a signal using speckle pattern imaging.

[0004] RF spectrometer based on a coherent light (often a laser) using speckle imaging provides a promising means to implement broad band real-time spectrum analysers with improved size, weight, power consumption requirement (SWaP) and cost characteristics over traditional implementations.

[0005] An example of a RF spectrum analyser that employs laser speckle imaging to characterise an RF signal is described in High-resolution broadband RF spectrometer based on laser speckle imaging, " Proc. SPIE 12225, Optics and Photonics for Information Processing XVI, 1222503 (3 October 2022); doi:

[0006] 10.1117 / 12.2631396.

[0007] A reference speckle pattern is generated by modulating an RF signal from the RF signal generator onto an optical carrier. This is repeated using different RF frequencies to generate a set of reference speckle patterns.

[0008] The set of reference speckle patterns can be used to characterise an unknown RF signal received through the antenna and modulated onto the optical carrier to produce a measurement speckle pattern.

[0009] The measurement speckle pattern can be considered as a superposition of multiple of the reference patterns or patterns extrapolations or interpolation thereof.

[0010] In order that the reference and measurement speckle patterns can be meaningfully compared, it is necessary for the system to be maintained in a stable condition during the entire period required for the reference speckle pattern and measurement speckle pattern to be obtained. Stabilising the system includes isolating components against vibration. To this end the prior art system pots the fibre in epoxy on an aluminium baseplate that is held in contact with a large optical table, and seals the fibre assembly in a box to isolate it from air currents. Additionally, it is necessary to stabilise the optical frequency of laser which is achieved using a feedback circuit including a rubidium cell. These stabilisation measures negate the otherwise promising SWaP and cost characteristics of the assembly.

[0011] Other examples of speckle pattern imaging RF spectrometers are described in:

[0012] High-resolution wavemeter based on polarization modulation of fiber speckles; APL Photonics 5, 126101 (2020); https: / / doi.org / 10.1063Z5.0028788;

[0013] Deep Learning Enabled Laser Speckle Wavemeter with a High Dynamic Range;

[0014] Roopam K. Gupta et al ; Laser Photonics Rev. 2020, 14, 2000120

[0015] According to a first aspect of the invention there is provided a speckle pattern-based radio frequency (RF) spectrum analyser system, said system comprising apparatus for speckle pattern capture, said apparatus comprising: an input for receiving a RF signal to be identified and / or characterised; means to generate an optical carrier modulated selectively with the RF signal received through the input; an optical dispersive element means configured to disperse the modulated optical carrier to produce a speckle pattern; a light sensing array configured to capture the speckle pattern; and a control system configured to :

[0016] (i)carry out a calibrating process comprising modulating the optical carrier to derive a set of reference speckle pattern data; and

[0017] (ii)carry out measurement process comprising modulating the optical carrier to impose the RF signal thereon to obtain a set of measurement speckle pattern data; the controller configured to control the RF signal generator and means to generate an optical carrier to repeat (i) and (ii) to generate a further set of reference speckle pattern data and a further set of measurement speckle pattern data; the system further comprising a signal processing means adapted to identify and / or characterise the RF signal from: the set of measurement speckle pattern data using a deconvolution process with the first set of reference speckle pattern data; and / or the one or more of the further sets of measurement speckle pattern data using a deconvolution process with one or more corresponding sets from the one or more sets of reference speckle pattern data.

[0018] A system may further comprise an RF signal generator to generate multiple RF calibration signals each of a different known frequency; means to generate an optical carrier modulated selectively with the RF signal received through the input or the RF calibration signals; wherein the control system is configured to control the RF signal generator and the means to generate the optical carrier, and wherein carrying out the calibrating process comprising modulating the optical carrier with a set of RF calibration signals of known frequencies to derive a set of reference speckle pattern data.

[0019] The invention may also be described in terms of a method and thus according to a second aspect of the invention there is provided a method to identify and / or characterise a RF signal with a speckle pattern imaging RF spectrometer system, the method comprising (i) collecting speckle pattern image data of the RF signal, and (ii) using a deconvolution process to identify and / or characterise the RF signal from the speckle pattern image data: wherein (i) comprises:

[0020] (a) carrying out a calibrating process comprising modulating an optical carrier to derive a set of reference speckle pattern data;

[0021] (b) modulating the optical carrier to impose the RF signal thereon to provide a modulated optical carrier and; obtaining a set of measurement speckle pattern data from the modulated optical carrier;

[0022] (c) repeating (a) and (b) to generate a further set of reference speckle pattern data; and corresponding further set of measurement speckle pattern data; wherein (c) is carried out periodically at a pre-determined repetition frequency, or dynamically in response to a determination that recalibration is required; and in which (ii) comprises:

[0023] (d) identifying and / or characterising the RF signal from: the first set of measurement speckle pattern data using a deconvolution process with the set of reference speckle pattern data; and / or from the further set of measurement speckle pattern data using a deconvolution process with the further set of reference speckle pattern data.

[0024] Carrying out a calibrating process may comprise modulating an optical carrier with a set of RF calibration signals of known frequencies to derive the set of reference speckle pattern data.

[0025] The following applies to either aspects of the invention.

[0026] Through application of the invention, the spectrometer is able to automatically recalibrate during monitoring of a RF signal to compensate for changes in the speckle patterns over time resulting from changes in environmental factors, such as temperature and vibration, that would otherwise lead to disassociation between the measurements speckle pattern images and consequent inability to deconvolve to identify and characterise the RF signal over the monitoring period.

[0027] By recalibrating frequently, many of the stabilising features of the prior art systems can be omitted, including those used to minimise vibration of the whole or parts of the apparatus including the fibre and / or modulator, as well as the feedback circuit used to stabilise the laser. This allows for significant reduction in the overall size of the apparatus.

[0028] The system is thus favourably adapted to recalibrate repeatedly at a repetition frequency equal or greater than the decorrelating effect of the noise. Expressed another way the period between recalibrations, i.e. period between instances of repeating (i) is shorter than the period of time taken for the effects of the noise on the patterns to become decorrelated.

[0029] How the frequently calibration is required depends on the environmental conditions experienced by the system whilst in use. In benign conditions, e.g. on a bench in a quiet laboratory, this could be as infrequency as once a minute; however, where used outdoors, handheld or on a moving platform where subjection to continuous vibration is likely, the calibration frequency would need to be higher than the mechanical vibration period; in this case 50millisecond or less is considered a suitable period between calibrations, though a period of 1ms or less may be more preferable for more hostile environments, such as expected on an aerial platform. Recalibration at frequencies high enough to compensate for vibration are likely to be sufficient to compensate for any changes in the temperature of the environmental or that of the laser.

[0030] The deconvolution process may be carried out simultaneously or subsequent to collection of the sets of reference and measurement patterns.

[0031] The signal processor may be co-located with the apparatus, e.g. in the same housing, but it is generally expected to be more preferable to locate the signal processor remotely to the apparatus.

[0032] There are a number of ways the recalibration signal can be generated. In its simplest form, the system would switch to calibration mode after a pre-determined time period identified from a clock signal. Thus the apparatus may comprise a clock configured to provide a clock signal, and the controller configured to use the clock signal to periodically repeat (i) and (ii) at a fixed repetition frequency, e.g. at a frequency equal or less than 100Hz. Alternatively, the apparatus may comprise one or more status sensors adapted to sense at least one of the following parameters: a temperature of the laser, acceleration of the apparatus (e.g. detected using a MEMS accelerometer sensor), rotation of the apparatus, and humidity of environment in which the apparatus is exposed; the one or more status sensors adapted to output signals indicative of the one or more parameters to the controller; and the controller configured to repeat (i) and (ii) in response to a determining from the output signals that recalibration is required.

[0033] An improved signal to instrument noise ratio and thus improved detection and characterisation of small RF signals is achieved with increasing number of measurement patterns used in the deconvolution process. Thus the dynamic process provides the advantage that calibration is only repeated when necessary, potentially allowing for capture and integration of a greater number of measurement speckle patterns. Nevertheless, it may also be possible to integrate RF signal data derived from each of multiple measurement speckle patterns from multiple different sets of measurement patterns.

[0034] The RF reference signal generator may be a tuneable signal generator configured to be swept to provide the reference signals. The means to generate the modulated optical carrier may comprise a coherent light source, e.g. a laser, to provide an optical carrier, and an electro-optic modulator means, e.g. Mach Zehnder modulator or a phase modulator, to modulate the optical carrier selectively with the RF signal received through the input or the RF calibration signals.

[0035] The effect of modulating an optical carrier with an RF signal is to produce a spectrum of optical frequencies. Rather than using a calibration RF signal to alter the optical carrier frequency, the light source could itself be controlled to do this.

[0036] As such the control system may configured to control the means to generate the optical carrier, and wherein carrying out the calibrating process comprises sweeping the optical frequency of the optical carrier to derive a set of reference speckle pattern data.

[0037] It is possible that the control system could be configured to derive a set of reference speckle pattern data by both sweeping the optical frequency of the optical carrier and imposing one or more RF calibration signals onto the optical carrier.

[0038] The invention will now be described by way of example with reference to the following Figures in which:

[0039] Figure 1 is a schematic of a speckle pattern-based radio frequency spectrum analyser system; Figure l is a flow diagram illustrating a process flow by which the system operates to recalibrate during operation; and

[0040] Figure 3 is a schematic of a variant speckle pattern-based radio frequency spectrum analyser system.

[0041] With reference to Figure 1 there is illustrated a speckle pattern based radio frequency spectrum analyser 1 comprising a laser 2, an electro-optical modulator 3, a scattering means 4, an image sensor 5, an analogue-to-digital converter 6, a signal processor 7 and a store 8.

[0042] The laser 2 is adapted to output a coherent beam of light 20, typically carried by an optical fibre, to an optical input 3 A of the electro-optical modulator 3. The beam 20 has a linewidth narrower than the linewidth of the side bands the spectrum analyser 1 is to resolve.

[0043] The electro-optical modulator 3 also comprises an electrical input 3B to receive a radio frequency signal, e.g. a signal above 1 kHz and including those into THzs. A suitable electro-optical modulator 3 is a Mach Zehnder modulator. Nevertheless, alternatives such as an acoustic optical modulator or a plasmonic modulator, polymer modulator or Indium Phosphide modulator may be used instead.

[0044] The modulator 3 functions to impose an RF signal received through its electrical input 3B onto the optical beam 20 to output a modulated optical carrier 21. The modulated carrier 21 is directed through the dispersive means 4, which scatters the light to form a speckle pattern on the image sensor array 5.

[0045] The dispersive element 4 may be implemented, for example, by one or more of: an optical scrambler; multi-mode fibre; integrating sphere; scattering screen; and transmission of the modulated beam 21 in free space through a gas, e.g. air. The image sensor 5 comprises an array of light sensors, which may be implemented using CCD or CMOS, adapted to capture image data of speckle patterns incident upon it. The captured image data is digitally converted by the ADC 6 and stored in store 8.

[0046] The analyser 1 further includes a controller system 9 providing a control and mode selector function 9A, a recalibration decision maker function 9B, and including calibration signal library 9C; a clock 10; a tuneable RF reference signal source 11, e.g. local oscillator, that outputs a RF reference signal of known frequency; a RF receiver and antenna 12 or other RF signal input means for receiving a RF signal to be identified and / or characterised; an amplifier 13; and a switch 14.

[0047] The library 9C holds a set of RF reference signal frequency values corresponding to a set of different RF frequencies covering the bandwidth of frequencies to be detected. The number of reference signal frequency values required depends upon a number of factors including: the bandwidth of the spectra to be detected and the desired accuracy required of the deconvolved output spectrum, which in part depends upon the dynamic range of the pixels of the array 5 namely, the intensity of the signal per pixel, amplitude of the signal per pixel and / or phase of the signal per pixel.

[0048] The control and mode selection function 9A is configured to switch operation of the analyser system 1 between two modes: a calibration mode, and a measurement mode. When operating in the measurement mode the system 1 collects and stores measurement speckle patterns derived from an RF signal to be characterised received through the antenna 12. In the calibration mode, the system 1 collects and stores reference speckle pattern data derived from the RF reference signals, which are used to process the measurement patterns to identify and / or characterise the RF signal. The control and mode selection function 9A switches from the measurement mode to the calibration mode in response to a recalibration signal from the recalibration decision maker 9B.

[0049] When switching to the calibration mode, the control and mode selection function 9A operates switch 14 to place a tuneable RF reference signal source 11 in communication with the MZM 3. The controller 9 selects a first calibration value from the library 9C and directs the RF reference signal source 11 to output a first RF reference signal of frequency corresponding to the first reference value. The MZM 3 modulates the optical carrier 20 from laser 2 to impose thereon the first RF calibration signal.

[0050] The process of modulating a RF signal onto the optical carrier 20 can be thought of as convolving the RF signal and optical carrier 20. When the optical carrier signal is modulated by a RF signal, optical side bands are created. The difference between the fundamental optical frequency and the side band frequencies are representative of the RF signal.

[0051] The modulated optical carrier 21 is dispersed by the dispersive means 4 and the resulting speckle pattern projected onto the image sensor array 5. Under control of the controller 9 the sensor 5 captures at least one image of the speckle pattern. The sensor array 5 outputs image data of the speckle pattern which is digitised by ADC 8 and the digital speckle pattern image data object stored in store 8 together with the associated calibration frequency signal value from controller 9 used to create it, and a time stamp derived from clock 10, as a first reference speckle pattern.

[0052] The controller 9 operates the RF signal source 11 to output a second RF calibration signal of a different frequency corresponding to a second calibration value held in library 9C. A digital speckle pattern image data object (RP) is recorded with associated calibration frequency signal value and time derived from clock 10 as a second reference speckle pattern. This is repeated to generate a separate reference speckle pattern for each calibration signal value held in library 9C hereafter referred to as a first set of reference speckle patterns (1stSRP). In a variant, rather than each reference speckle pattern having a time stamp, the first set of reference speckle patterns as a whole may have a single time stamp.

[0053] Once the controller 9 determines that the first set of reference speckle patterns has been captured, the control & mode selection function 9A switches operation of the system to the measurement mode in which the switch 14 is operated to place the antenna 12 in communication with the MZM 3.

[0054] RF signals to be identified and / or characterised (e.g. to determine the presence or not of one or more signals of known frequency and / or to identify the presence of signals of unknown frequencies) received through antenna 12 are modulated by the MZM 3 onto the optical carrier 20 and the resulting speckle pattern image data object stored in store 6A with time stamp as a first measurement speckle pattern. Typically the image sensor 5 will, under control of the controller 9, operate to capture multiple speckle pattern images in succession for as long as the system 1 is operating in measurement mode to generate multiple measurement speckle patterns, in other words a time series of measurement speckle patterns, herein after referred to as a first set of measurement speckle patterns (1stSMP). In a variant, rather than each measurement speckle pattern comprising a time stamp, a single time stamp may be associated with the set of measure speckle patterns as a whole.

[0055] Upon generation of the recalibration signal by the recalibration decision maker 9B, the control and mode selection function 9A reverts operation of the system 1 back to the calibration mode to collect and store in store 8 a further set of reference speckle patterns (2ndSRP) data derived from the calibration signals, where after it switches back to measurement mode to collect and store a further set of measurement speckle patterns (2ndSMP) until the next recalibration signal is generated.

[0056] For as long as the system 1 is set to monitor RF signals received through the antenna 12, the system 1 continues to switch between the calibration and measure modes so as to collect more sets of reference pattern data and more sets of measurement speckle pattern data.

[0057] The recalibration decision maker 9B is configured, using a clock signal from clock 10, to periodically generate the recalibration signal. The period length between recalibration signals may be pre-set at manufacture based on the most adverse environmental conditions the system 1 is anticipated to experience during operation. Specifically, the period length selected is short enough to ensure all speckle patterns captured between instances of recalibration, both reference and measurement, correlate to allow for deconvolution (described further below) to identify and / or characterise the RF signal.

[0058] In general, changes in temperature and humidity of the environment and changes in temperature of the laser 2 occur relatively slowly, in the order of second, minutes or hours, whereas decorrelation as a consequence of motion, e.g. vibration occurs much more quickly.

[0059] As such, a period length is selected based upon the highest frequency vibrations to which the system 1 is anticipated to be exposed. To maximise signal to noise ratio, the period length between recalibration signals is preferably at least ten times shorter than the period of the highest expected frequency vibration. For example, where one or more of the laser 2, MZM 3 and imager 5 are anticipated to be exposed to vibrations of up to around 2kHz during operation (e.g. where the apparatus is expected to be mounted onto a fast moving platform), the recalibration decision maker 9B may be configured to generate a recalibration signal at a repetition frequency of at least 20kHz corresponding to a maximum period length between recalibrations signals of fifty micro seconds. Where the apparatus is expected to be hand held when in use, the frequency of vibration may be substantially lower and thus the maximum period between vibrations may be greater, for example up to one millisecond.

[0060] Other factors may make selection of a longer period length preferred, such as the number of reference patterns and measurements patterns desired between calibrations, which itself may be dependent on the integration time required by the sensor array 5 to provide an acceptable signal to noise ratio.

[0061] It may be possible to use a longer period and compensate for noise caused by vibration during processing of the captured patterns such as, for example, by using the deep learning processing disclosed in Wen Xiong et al. Deep learning of ultrafast pulses with a multimode fiber. APL Photonics 5, 096106 (2020). The period required to capture the necessary reference speckle patterns in a single instance of the system operating in the calibration mode will depend upon the number of reference speckle patterns to be captured. It is favourable for the calibration mode to be as short as possible to maximise the time the system is operating in the monitoring mode. In a non-limiting example, the time to capture reference patterns may be in the region of 10ms with the remaining 40ms used to capture measurement patterns of the RF signal.

[0062] Where the system 1 is expected to only operate in very benign environments, not subject to meaningful vibration, the period length may be much longer. Where so a period of up to sixty seconds may be sufficient to compensate for slowly changing variables such as the changing temperature of the laser.

[0063] Deconvolution

[0064] The signal processor 7 is configured to identify and / or characterise the RF signal using one or more of the captured measurement speckle patterns from a selected set of the multiple sets of measurement speckle patterns held in store 8 with the corresponding set of reference speckle pattern held in the store; that being the set of reference speckle patterns captured immediately prior to the selected measurement speckle patterns. For example, one or more of the first set of measurement speckle patterns (1stSMP) with the first set of reference patterns (1stSRP), or one or more of the nth set of measurement speckle patterns (Nth SMP) in conjunction with the second set of reference patterns (Nth SRP).

[0065] First, the selected set of reference patterns are used to train a machine learning model, or transfer learn a previously trained machine learning model (MML). Thereafter, one or more of the measurement speckle pattern of the corresponding set of measurement speckle patterns is inputted to the model to determine outputs characteristics of the RF signal, e.g. one or more of frequency, QAM, phase modulation and amplitude modulation. The output characteristics derived from measure speckle patterns from multiple sets of measurement speckle patterns may be used together, e.g. averaged, to improve the signal to noise ratio.

[0066] Example MML models suitable for this application include: a convolution neural network, and linear regression model.

[0067] Alternative methods to deconvolve may be used instead of a machine learning model. These include regression analysis techniques including lasso regression such as described in Matthew J. Kelley et al, cited on page one; or a simulated annealing algorithm such as described in All-Fibre Spectrometer base on Speckle Pattern Reconstruction ; Brandon Redding et al Yale University, Optical Express 6584 vol 21 no. 5, 11 March 2013.

[0068] Other than the selection of which measurement data and reference data to use for the deconvolution, i.e. using corresponding sets, the method used to deconvolve is not important for the purposes of the invention.

[0069] Deconvolution may be carried out subsequent to collection of all of the reference and measurement data sets, or as is expected to be more common, in parallel with the capture of further sets of reference and measurement reference speckle patterns sets.

[0070] Multiple measurement speckle patterns from the same set of measurement speckle patterns may be combined (e.g. averaged) before being inputted into the model to improve signal to noise ratio

[0071] The processing requirement for carrying out deconvolution is fairly high so the signal processor 7 may be located remotely from the laser, modulator and image sensor. For example the signal processor function may be implemented by a remote server. Where so, the system 1 may also include a transmitter (e.g. a wireless transmitter) to transmit the collected data in store 8, e.g. through the internet and / or cellular network, to the signal processor 7. The system 1 may also include a receiver (e.g. a wireless receiver) to receive RF signal characterising information resulting from the deconvolution back from the signal processor 7. It may also include a user interface to communicate said RF signal characterising information to a user of the system 1. In variants where the signal processor 7 is located remotely, it may be used as a shared resource to carry out deconvolution for multiple speckle pattern based radio frequency spectrum analyser systems.

[0072] An advantage of using a remote processor to implementing the signal processor 7 is that it significantly reduces the physical size and cost of the rest of the system, allowing for example, it to be manufactured as a hand-held device.

[0073] Variants

[0074] Instead of generating the recalibration signal repeatedly at a fixed repetition frequency, the switch from measurement mode to calibration mode may instead be made dynamically in response to detected changes in the environment and / or of system status, e.g. temperature of the laser 2, movement of the system 1.

[0075] For example, the system 1 may include one or more environmental and / or system status sensors e.g. to detect one or more of, a temperature (e.g. environmental and / or of the laser 2), acceleration of the system, rotation of the system, and humidity. The recalibration decision maker 9B is configured to generate a recalibration signal when determined from a signal from one or more of the environmental sensors indicative of a change in the assembly or environment that exceeds a threshold amount and thus warrants recalibration.

[0076] In another variant, the recalibration decision maker 9A may comprise a comparator configured to compare two or more measurement speckle patterns within a set and output a difference value indicative of said difference between the patterns. For example, the comparator may be configured to compare one or more pixel values between different frames, where a significant difference in the two frames may indicate a uncontrolled change, caused e.g. by vibration, and thereby warrants recalibration. Where so, the control & mode selector function 9A may be configured to switch from measurement mode to calibration mode when the difference value exceeds a threshold value.

[0077] Because it simplifies operation, it is preferred that for a given set of reference speckle patterns and set of measurement speckle patterns used for a deconvolution process, the set of reference speckle patters were captured immediately prior to said set of measurement patterns. Nevertheless, this is not essential.

[0078] In a further variant, the library 9C and tuneable RF signal source 11 may be replaced by a bank of signal sources each configured to generate a different one of the known RF reference frequencies, and each individually selectively connectable to the electrooptic modulator 3 through switch 14. When in calibration mode, the controller 9 is configured to connect each of the signal sources to the modulator 3 in turn to produce a set of reference speckle patterns.

[0079] The system may use methods other than time stamps to group reference and measurement images for deconvolution. For example, rather than associating each image with a time stamp, each image may instead be associated with a counter value which increments by one each time the system switches from measurement mode to calibration mode.

[0080] Using a laser to produce the optical carrier and a separate electro-optical modulator, such as an MZM or a phase modulator, to modulate the optical carrier is likely to be the preferred means of providing a modulated optical carrier, particularly for microwave wavelength RF signal, however alternatives are possible. For example, alternative light sources may be used to generate the optical carrier such as, for example, an incandescence neon lamp. Further, the light source may be directly modulated by the RF signal (calibration or measurement), i.e. exposed to the RF signal meaning a separate modulator is not needed. Both a laser and a lamp are capable of directly modulating their output within certain RF frequency bands, in the case of the laser this is likely to be limited to modulating frequencies below 5GHz, and can be achieved through changing the temperature of the laser, or more preferably varying the injection current.

[0081] The principle of the invention may also be applied to a variant spectrum analyser in which the optical beam 20 is a multiplexed beam derived from multiplexing multiple optical beams of different, non-overlapping, wavelengths from a set of lasers. The multiplexed beam is modulated by the modulator 3 and dispersed by the dispersive means 4 as described above before being split by a wavelength dependent splitter back into its constituent beams each of which being directed onto a separate imager 5 to produce a separate speckle pattern. In this way multiple temporally aligned speckle patterns are produced each representative of the RF signal (whether it be reference or received through antenna) depending which is received by the modulator 3. This method produces speckle patterns at a greater rate that may allow for quicker and / or improved detection of small signals with the deconvolving process.

[0082] Generalised Method of Speckle Pattern Data Collection

[0083] Figure 2 illustrates the general process flow that the system follows whilst monitoring for RF signals received through antenna 12.

[0084] The system 1 is initiated to monitor for RF signals

[0100] , The controller 9 enters the calibration mode

[0101] and connects the RF reference signal generator to the modulator 3. The system captures and stores a first set of reference speckle patterns

[0102]

[0085] The controller 9 then switches to measurement mode

[0103] whereupon the antenna 12 is connected to the modulator 3. The system 1 collects and stores one or more measurement speckle pattern

[0104] of a first set of measurement speckle patterns. The controller 9 determines whether recalibration is required. If not, the repeats step

[0104] to take a further one or more measurement speckle patterns to add to the first set of measurement speckle patterns. If it is determined that recalibration is required, the controller 9 reverts to calibration mode

[0101] and repeats steps

[0010] 2-

[0104] to capture a second set of reference speckle patterns and a second set of measurement speckle patterns.

[0086] This cycle is repeated until the system 1 is instructed, e.g. through a user inputted signal, to stop monitoring RF signals.

[0087] Figure 3 illustrates a further variant system 100 having a manner of operation similar to that of the system of Fig 1 but rather than modulating the optical carrier with known RF frequencies to generate the reference speckle patterns, the reference speckle patterns are instead generated by altering the wavelength of operation of the laser 2’ in a controlled manner.

[0088] The laser 2’ of the system 100 is tuneable and its wavelength of operation is controlled by control signals received from the controller 9.

[0089] When in the calibration mode the control and mode selection function 9A operates switch 14 to disconnect the electro-optic modulator 3 from the RF signal input 12. The controller 9 selects a first calibration value from the library 9C and directs the tuneable laser 2 to output a beam of light of a first optical wavelength corresponding to the first calibration value held in library 9C.

[0090] As before, the optical carrier 21 is dispersed by the dispersive means 4 and the resulting speckle pattern projected onto the image sensor array 5. Under control of the controller 9 the sensor 5 captures at least one image of the speckle pattern. The sensor array 5 outputs image data of the speckle pattern which is digitised by ADC 8 and the digital speckle pattern image data object stored in store 8 together with the first calibration value from controller 9 used to create it, and a time stamp derived from clock 10, as a first reference speckle pattern. The controller 9 operates the laser 2’ to output an optical beam of a second optical frequency corresponding to a second calibration value held in library 9C. A digital speckle pattern image data object (RP) is recorded with associated second calibration value and time derived from clock 10 as a second reference speckle pattern. This is repeated to generate a separate reference speckle pattern for each calibration signal value held in library 9C hereafter referred to as a first set of reference speckle patterns (1stSRP). As before, in a variant, rather than each reference speckle pattern having a time stamp, the first set of reference speckle patterns as a whole may have a single time stamp.

[0091] Once the controller 9 determines that the first set of reference speckle patterns has been captured, the control & mode selection function 9A switches operation of the system to the measurement mode in which the switch 14 is operated to place the antenna 12 in communication with the MZM 3.

[0092] To identify and / or characterised RF signals received through input 12, the controller switches to measurement mode. The control and mode selection function 9A operates switch 14 to place the electro-optic modulator 3 in communication with the RF signal input 12. A set of measure speckle patterns can then be obtained in the manner afore described.

Claims

Claims1. A speckle pattern-based radio frequency (RF) spectrum analyser system, said system comprising apparatus for speckle pattern capture, said apparatus comprising: an input for receiving a RF signal to be identified and / or characterised; means to generate an optical carrier modulated selectively with the RF signal received through the input; an optical dispersive element means configured to disperse the modulated optical carrier to produce a speckle pattern; a light sensing array configured to capture the speckle pattern; and a control system configured to control the RF signal generator and the means to generate the optical carrier to:(i)carry out a calibrating process comprising modulating the optical carrier to derive a set of reference speckle pattern data; and(ii)carry out measurement process comprising modulating the optical carrier to impose the RF signal thereon to obtain a set of measurement speckle pattern data; the controller configured to control the RF signal generator and means to generate an optical carrier to repeat (i) and (ii) to generate a further set of reference speckle pattern data and a further set of measurement speckle pattern data;the system further comprising a signal processing means adapted to identify and / or characterise the RF signal from: the set of measurement speckle pattern data using a deconvolution process with the first set of reference speckle pattern data; and / or the one or more of the further sets of measurement speckle pattern data using a deconvolution process with one or more corresponding sets from the one or more sets of reference speckle pattern data.

2. A system according to claim 1 further comprising an RF signal generator to generate multiple RF calibration signals each of a different known frequency; means to generate an optical carrier modulated selectively with the RF signal received through the input or the RF calibration signals; wherein the control system is configured to control the RF signal generator and the means to generate the optical carrier, and wherein carrying out the calibrating process comprising modulating the optical carrier with a set of RF calibration signals of known frequencies to derive a set of reference speckle pattern data.

3. A system according to claim 1 or 2 wherein the control system is configured to control the means to generate the optical carrier, and wherein carrying out the calibrating process comprises sweeping the optical frequency of the optical carrier to derive a set of reference speckle pattern data.

4. The system of claim 1, 2 or 3 wherein the apparatus comprises a clock configured to provide a clock signal, and the controller is configured to use the clock signal to periodically repeat (i) and (ii).

5. The system of any claim 1-4 wherein the apparatus comprises a status sensor adapted to sense at least one of the following parameters: acceleration of the apparatus, rotation of the apparatus, humidity and / or temperature of environment in which the apparatus is exposed; the status sensor adapted to output signals indicative of the one or more parameters to the controller; the controller configured to repeat (i) and (ii) in response to determining from the outputs signals that recalibration is required.

6. A system according to any claim 1,2 4-6 wherein the means to generate the optical carrier comprises a light source to provide an optical carrier and an electro-optic modulator means configured to modulate the optical carrier selectively with the RF signal received through the input or the RF calibration signals.

7. A system according to any claim 3-5 wherein the means to generate the optical carrier comprises a laser and a control system is configured to the laser to sweep the optical frequency of the optical carrier .

8. A method to identify and / or characterise a RF signal with a speckle pattern imaging RF spectrometer system, the method comprising (i) collecting speckle pattern image data of the RF signal, and (ii) using a deconvolution process to identify and / or characterise the RF signal from the speckle pattern image data: wherein (i) comprises:(a) carrying out a calibrating process comprising modulating an optical carrier with a set of RF calibration signals of known frequencies to derive a set of reference speckle pattern data;(b) modulating the optical carrier to impose the RF signal thereon to provide a modulated optical carrier and; obtaining a set of measurement speckle pattern data from the modulated optical carrier;(c) repeating (a) and (b) to generate a further set of reference speckle pattern data; and corresponding further set of measurement speckle pattern data; wherein (c) is carried out periodically at a pre-determined repetition frequency, or dynamically in response to a determination that recalibration is required; and in which (ii) comprises:(d) identifying and / or characterising the RF signal from: the first set of measurement speckle pattern data using a deconvolution process with the set of reference speckle pattern data; and / or from the further set of measurement speckle pattern data using a deconvolution process with the further set of reference speckle pattern data.

9. A method according to claim 8 wherein carrying out the calibrating process comprises modulating the optical carrier with a set of RF calibration signals of known frequencies to derive the set of reference speckle pattern data.

10. A method according to claim 8 or 9 wherein carrying out the calibrating process comprises sweeping the optical frequency of the optical carrier to derive a set of reference speckle pattern data11. A method according to any claim 8-10 wherein (c) is carried out periodically at a pre-determined repetition frequency equal or greater than once a minute.

12. A method according to claim 11 wherein (c) is carried out periodically at a predetermined repetition frequency equal or greater than once a millisecond.

13. A method according to any claim 8-11 wherein (c) is carried out dynamically in response to one or more of: an output from one or more status sensor indicative of at least one of the following parameters: acceleration of the apparatus, rotation of the apparatus, and humidity and / or temperature of environment in which the apparatus is exposed; an output from a comparator indicative of a difference between two of more reference speckle patterns.

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