Method of generating time domain echo waveforms and electromagnetic radiation echo waveform generation system

Abstract

Methods of generating time domain echo waveforms and electromagnetic radiation echo waveform generation systems are provided herein. The method of generating time domain echo waveforms includes a trigger source that emits a plurality of pulses of electromagnetic radiation. A plurality of reflected pulses irradiate an electromagnetic radiation detector unit, which generates a plurality of stored electrical measurements in response thereto. The method further includes generating a time-varying mixing signal and applying a phase-shifted version of the time-varying mixing signal to the detector separately while the detector generates the plurality of electrical measurements. A signal preprocessor reads out the plurality of electrical measurements from the detector. A signal reconstruction unit then generates a frequency spectrum of the electrical measurements and a frequency spectrum of the mixing signal. The signal reconstruction unit generates a reconstructed signal spectrum by deconvolving the frequency spectrum of the stored electrical measurements using the frequency spectrum of the mixing signal and then generates an echo waveform by converting the reconstructed signal spectrum to the time domain.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of generating a time domain echo waveform, e.g. of the type that converts a reflected electromagnetic radiation pulse incident on an electromagnetic radiation detector cell into an electrical waveform. The present invention also relates to an electromagnetic radiation echo waveform generation system, e.g. of the type that generates an electrical waveform in response to irradiating an electromagnetic radiation detector cell with a reflected electromagnetic radiation pulse. BACKGROUND

[0002] In so-called time-of-flight sensing systems and other systems, e.g. night vision systems, it is known to employ an illumination source to illuminate and process light reflected by features of a scene, sometimes referred to as the "field of view", within the field of view of the illumination source. Such so-called LiDAR (Light Detection and Ranging) systems use an illumination source to illuminate a scene with light and a detection device, e.g. an array of photodiodes, some optical elements, and a processing unit, to detect light reflected from objects in the scene. The light reflected from objects in the scene is received by the detection device and converted into an electrical signal which is then processed by the processing unit by applying a time-of-flight (ToF) calculation in order to determine the distance of the objects from the detection device. Although different kinds of LiDAR systems are known based on different working principles, such systems basically illuminate a scene and detect reflected light.

[0003] In this regard, so-called "flash LiDAR" technology is a direct ToF ranging technology that employs a light source that emits a light pulse that is subsequently reflected by a scene feature and detected by a detector device. In such technology, the distance to a reflecting feature is directly calculated using the measured time of the round trip of the light pulse to the reflecting feature and back to the detector device. The light pulse incident on the detector device is sampled in the time domain at a very high sampling rate. Therefore, the signal path in the processing circuitry implementing such technology requires high bandwidth of the signal as well as large silicon "real estate", i.e. such implementations require a relatively large area on the silicon wafer which in turn limits the number of channels that can be supported on the integrated circuit. Therefore, the actual number of spatial channels that can be supported by such flash LiDAR sensors is typically below 100. In order to overcome this limitation, mechanical scanning systems are required that move components.

[0004] Another known LiDAR system employs so-called "indirect time-of-flight" (iToF) ranging technology. iToF systems emit a continuous wave light signal and reflections of the continuous wave light signal are received by a detector device and analyzed. Multiple samples, e.g. four samples, of the light reflected from a feature of a scene are taken, each sample phase stepped 90°. Using this illumination and sampling method, the phase angle between the illumination and the reflection can be determined and the determined phase angle can be used to determine the distance to the reflecting feature of the scene.

[0005] In an iToF system, high frequency signal processing (demodulation) occurs at the pixel level, so the post-pixel signal bandwidth required to integrate a large number of pixels on the same chip is low. Therefore, compared to a direct ToF system, an iToF system is able to support a larger number of channels, thereby supporting a higher spatial resolution measurement. However, the measurement distance capability of an iToF system is limited. In this regard, to achieve a low random distance measurement error, an iToF system requires a high modulation frequency, which in turn reduces the range of distances that can be measured unambiguously. For example, a modulation frequency of 100 MHz results in an unambiguous measurement distance of approximately 1.5 m. Also, conventional iToF systems are susceptible to errors due to multiple reflections and multiple propagation paths. SUMMARY

[0006] According to a first aspect of the application, there is provided a method of generating a time domain echo waveform, the method comprising: a trigger source of pulsed electromagnetic radiation emitting a plurality of pulses of electromagnetic radiation in response to a trigger signal; a plurality of electromagnetic radiation reflection echo signals irradiating an electromagnetic radiation detector cell, the electromagnetic radiation detector cell generating a plurality of stored electrical measurements in response to electromagnetic radiation incident thereon; generating a time varying mixing signal; applying a phase shifted variant of the mixing signal to the electromagnetic radiation detector cell respectively whilst the plurality of stored electrical measurements are being generated by the electromagnetic radiation detector cell; a signal pre-processor reading out the plurality of stored electrical measurements from the electromagnetic radiation detector cell; a signal reconstruction unit generating a frequency spectrum of the plurality of stored electrical measurements and providing a frequency spectrum of the mixing signal; the signal reconstruction unit generating a reconstructed signal frequency spectrum by deconvolving the frequency spectrum of the plurality of stored electrical measurements using the frequency spectrum of the mixing signal; and the signal reconstruction unit generating the time domain echo waveform by converting the reconstructed signal frequency spectrum to the time domain.

[0007] The time varying function can be binary, but higher logic levels can be employed, for example, tristate logic. The time varying function can be N bits in length. The time varying mixing signal can be a pseudo random binary sequence (PRBS) signal.

[0008] Deconvolving the frequency spectrum of the plurality of accumulated electrical measurements can comprise inverse filtering the frequency spectrum of the plurality of stored electrical measurements with the frequency spectrum of the mixing signal.

[0009] The time varying mixing signal can be a periodic signal.

[0010] Deconvolving the frequency spectrum of the plurality of stored electrical measurements can comprise Wiener deconvolution of the frequency spectrum of the plurality of stored electrical measurements with the frequency spectrum of the mixing signal.

[0011] The phase shifted variants of the mixing signal can be generated by phase shifting the mixing signal serially.

[0012] The photodetector cell can be a photomixer device.

[0013] The photomixing device can comprise a modulation electrode; the method can further comprise applying a phase-shifted version of the mixing signal to the modulation electrode.

[0014] The method can further comprise providing a time-varying mixing signal generator to apply a phase-shifted version of the mixing signal to the electromagnetic radiation detector unit respectively; generating a trigger signal; applying the trigger signal to a trigger source of the pulsed electromagnetic radiation; and applying the trigger signal to the time-varying mixing signal generator substantially simultaneously.

[0015] According to a second aspect of the present application, there is provided a method of measuring a distance to a reflecting object, comprising: detecting a reflected pulse of electromagnetic radiation by analysing a time-domain echo waveform generated using the method of generating a time-domain echo waveform in relation to the first aspect of the present application as described above.

[0016] The method can further comprise calculating the distance in respect of the reflected pulse of the plurality of reflected electromagnetic pulses using the detected reflected echo signal.

[0017] Generating a stored electrical measurement of the plurality of stored electrical measurements can comprise: a predetermined number of the plurality of electromagnetic radiation reflected pulses being incident on the electromagnetic radiation detector unit in succession and the electromagnetic radiation detector unit generating a corresponding number of respective electrical measurements in response thereto respectively; applying a variation of the phase-shifted version of the mixing signal to the electromagnetic radiation detector unit whilst the electromagnetic radiation detector unit generates the corresponding number of respective electrical measurements; and accumulating the corresponding number of respective electrical measurements.

[0018] The stored electrical measurement of the plurality of stored electrical measurements can be in respect of a number of the plurality of electromagnetic radiation reflected echo signals. The number of echo signals can be one echo signal.

[0019] The method can further comprise: providing a plurality of electromagnetic radiation detector units comprising the electromagnetic radiation detector unit; applying each variation of the phase-shifted version of the mixing signal to the plurality of electromagnetic radiation detector units whilst each of the plurality of electromagnetic radiation detector units generates a plurality of stored electrical measurements in respect of each variation of the phase-shifted version of the mixing signal respectively; the signal pre-processor reading out the stored electrical measurements from the plurality of electromagnetic radiation detector units substantially in parallel respectively, each read-out being in respect of each variation of the phase-shifted version of the mixing signal; the signal pre-processor aggregating the electrical measurements read out from the plurality of electromagnetic radiation detector units in respect of each parallel read-out, thereby generating a plurality of aggregated stored electrical measurements; the signal reconstruction unit generating a frequency spectrum in respect of the plurality of aggregated stored electrical measurements and generating a frequency spectrum of the mixing signal; the signal reconstruction unit generating a reconstructed signal frequency spectrum by deconvolving the frequency spectrum of the plurality of aggregated stored electrical measurements using the frequency spectrum of the mixing signal; and the signal reconstruction unit generating a time-domain echo waveform by converting the reconstructed signal frequency spectrum to the time domain.

[0020] The time domain echo waveform can be a representation in the electrical domain of a plurality of electromagnetic radiation reflection pulses incident on the electromagnetic radiation detector unit. The pulsed source of electromagnetic radiation can generate a pulsed output, rather than a continuous wave output.

[0021] According to a third aspect of the present application, there is provided an electromagnetic radiation echo waveform generation system, comprising: a pulsed source of electromagnetic radiation configured to emit a plurality of electromagnetic radiation pulses in response to a trigger signal; an electromagnetic radiation detector unit configured to generate a plurality of stored electrical measurements in response to a plurality of electromagnetic radiation reflection echo signals irradiating the electromagnetic radiation detector unit; a time varying mixing signal generator configured to apply a phase shifted variant of a mixing signal to the electromagnetic radiation detector unit while the electromagnetic radiation detector unit generates the plurality of stored electrical measurements; and a signal pre-processor configured to read out the plurality of stored electrical measurements from the electromagnetic radiation detector unit; wherein a signal reconstruction unit is configured to generate a frequency spectrum of the plurality of stored electrical measurements and to generate a frequency spectrum of the mixing signal; the signal reconstruction unit is configured to generate a reconstructed signal spectrum by deconvolving the frequency spectrum of the plurality of stored electrical measurements using the frequency spectrum of the mixing signal; and the signal reconstruction unit is configured to generate a time domain echo waveform by converting the reconstructed signal spectrum to the time domain.

[0022] Thus, it is possible to provide a method of generating a time domain echo waveform and an electromagnetic radiation echo waveform generation system which support a greater range of measurements of a direct ToF system under the lower bandwidth requirements of an iToF system. Furthermore, the method and system are able to reconstruct the time domain echo waveform and are able to remove multiple reflections and / or multiple propagation paths. BRIEF DESCRIPTION OF DRAWINGS

[0023] At least one embodiment of the present application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0024] Figure 1 is a schematic diagram of a distance measurement system arranged in a scene and comprising an electromagnetic radiation detector device constituting an embodiment of the present application;

[0025] Figure 2 is Figure 1 is a flow diagram of a method of generating a time domain echo waveform used by an embodiment of

[0026] Figure 3 is a flow diagram of a reconstruction process of Figure 2 in more detail and constituting a further embodiment of the present application;

[0027] Figure 4 is a diagram of a measurement light signal and a mixing signal used in an embodiment of Figures 1 to 3 ​

[0028] Figure 5 is a spectrogram of the measured optical signal, the mixed signal and the reconstructed signal;

[0029] Figure 6 is a plot comparing the recovered echo waveform and the received optical signal when the method of Figure 2 and Figure 3 is a plot comparing the recovered echo waveform and the received optical signal when the method of

[0030] Figure 7 is a flowchart of a further reconstruction process of Figure 2 and constitutes a further embodiment of the invention;

[0031] Figure 8 is a further measured optical signal and a further mixed signal used according to the embodiments of Figure 1 , 2 and 7;

[0032] Figure 9 is a spectrogram of the measured optical signal and the mixed signal of Figure 8 ; and

[0033] Figure 10 is a plot comparing the recovered echo waveform and the received optical signal when the method of Figure 2 and Figure 7 is a plot comparing the recovered echo waveform and the received optical signal when the method of

[0034] Throughout the following description identical reference signs will be used to identify identical parts. DETAILED DESCRIPTION

[0035] With reference to Figure 1 , an electromagnetic radiation ranging system (e.g. a pulsed light detection and ranging system 100) is arranged, for example, within an environment to monitor a so-called scene 102. A typical application of the system 100 is in a LIDAR system. The scene 102 comprises a reflecting object 104.

[0036] The system 100 includes a detection circuit 106 and an illumination source, e.g., a pulsed optical source or light source 108 (such as a pulsed laser or laser diode). In this regard, the electromagnetic radiation source 108 provides a pulsed output, as opposed to a continuous wave output, the distinction being apparent to those skilled in the art. Although not shown, in this example, the light source 108 includes optics and / or filters for modifying the optical energy emitted by the light source 108. A pulse generator 110 is operatively coupled to the light source 108 and to a signal generator 112, e.g., a time-varying binary signal generator (such as a pseudo-random binary sequence (PRBS) signal generator), which is operatively coupled to a phase selector unit 114. In this example, the signal generator 112 is operatively coupled to a photodetector unit 116 through a modulation electrode 118 of a photonic mixer 124. The photonic mixer 124 can be any suitable kind of grating device, e.g., of the type described in “3D Camera Based On Gain-Modulated Cmos Avalanche Photodiodes” (O. Shcherbakova, PhD Thesis, April 2013, https: / / core.ac.uk / download / pdf / 35317180.pdf). Although not shown, it will be appreciated that the detection circuit 106 includes an array of photodetector units, although for clarity and conciseness of illustration, only the photodetector unit 116 is shown and primarily described herein. An optical system (also not shown), including, e.g., a lens (such as a focusing lens), can also be provided in the vicinity of the photonic mixer array of the detection circuit 106.

[0037] The photodetector unit 116 includes a signal accumulation unit, for example an integrator 120 having an input operably coupled to the photomixer device 124 controlled by the signal generator 112. The output of the integrator 120 is operably coupled to a signal pre-processor, for example a signal preparation and conditioning circuit 126, the output of which is operably coupled to an input of a signal reconstruction unit 128. The signal preparation and conditioning circuit 126 is also operably coupled to the pulse generator 110. The signal preparation and conditioning circuit 126 typically includes amplification circuitry, analogue to digital conversion circuitry, and optionally low pass filtering to deal with aliasing during analogue to digital conversion, in order to generate an output signal that can be processed by the signal reconstruction unit 128 and a microprocessor 130, which is operably coupled to the signal reconstruction unit 128. Optionally, the signal reconstruction unit 128 can apply additional filtering, for example optimal filtering, low pass filtering or Gaussian filtering. The microprocessor 130 constitutes a processing resource and is typically supported by non-volatile memory, for example read only memory, and volatile memory, for example random access memory (not shown). Although in this example the pulse generator 110 is coupled to the signal preparation and conditioning circuit 126, it will be appreciated by the skilled person that synchronisation can be achieved by coupling the pulse generator 110 to other functional elements of the system 100, for example the microprocessor 130.

[0038] The microprocessor 130 supports a number of functional units, for example a pulse analyser (not shown) in order to determine distances to a reflecting object, for example the object 104. The pulse analyser can implement any suitable signal processing technique to obtain such distance information, for example as described in co-pending European patent application number 18165668.7, the contents of which are incorporated herein by reference.

[0039] In operation, the signal reconstruction unit 128 deconvolves the electrical measurement provided at the output of the integrator 120 and processed by the signal preparation and conditioning circuit 126, which has been pre-processed with the phase variable mixing signal provided by the signal generator 112, in order to recover the analogue echo waveform.

[0040] The pulsed light detection and ranging system 100 relies on a cross-correlation function in order to obtain a cross-correlation term by applying a phase shifted version of the mixing signal to the photocurrent generated by the photomixer device 124. In this respect, a photocurrent is generated in response to light incident on the photomixer 124 and the charge associated with the photocurrent is subsequently stored in the integrator 120, or in the case of an array of photodetector units 116, the cumulative stored charge of the individual photodetector units 116 is considered. The cross-correlation function is:

[0041]

[0042] where is the cross-correlation term, o(t) is the photocurrent generated in response to the received reflected light signal, and m(t) is the mixing or demodulation function.

[0043] The above expression can be rewritten as the following convolution:

[0044]

[0045] where o is the photocurrent generated in the time domain in response to the incident light signal, and m is the mixing signal in the time domain. To recover the electrical measurement of the incident light signal from the cross-correlation term , the above equation (2) can be used in the frequency domain in conjunction with deconvolution. In this regard, to deconvolve the above convolution, a Fast Fourier Transform (FFT) needs to be performed on the electrical measurement signal that constitutes the cross-correlation term and the mixing signal m:

[0046]

[0047] Using these Fourier transforms, equation (2) can be simply adjusted to implement a basic deconvolution:

[0048]

[0049] where O is the frequency spectrum of the electrical measurement of the incident light signal. To convert the frequency domain representation of the electrical measurement to the time domain, an Inverse FFT (IFFT) is performed on the frequency spectrum of the deconvolved cross-correlation term : O = IFFT(O)

[0050]

[0051] Another known deconvolution technique can be employed instead of equation (3), namely, Wiener deconvolution:

[0052]

[0053] where K is a function defined by the noise spectrum.

[0054] As in the case of equation (3), the electrical measurement in the time domain of the received light signal can be obtained by performing an IFFT on the frequency spectrum O of the electrical measurement of the received light signal.

[0055] Reference is made to Figure 2When the system 100 is powered on, the phase selector unit 114 initializes the phase shift counter ph to zero (step 200). The pulsed light source 108 emits light pulses 132 (step 202), for example in response to (i.e., triggered by) the pulse generator 110, which illuminate the scene 102 and are incident on the reflecting object 104. The trigger signal generated by the pulse generator 110 is also delivered substantially simultaneously to the signal generator 112 and the signal preparation and conditioning unit 126. The light 132 incident on the object 104 is reflected by the object 104, the amount of reflected light 132 depending on the reflectivity of the object 104. The reflected light 134 is received by the array of photodetectors 116 and converted to the electrical domain in the form of a time-varying analog output signal that is proportional to the intensity of the light received by the array of photodetectors 116 over a period of time. The reflected light 134 is received by the array of photodetector units 116 and converted to the electrical domain in the form of a time-varying analog output signal that is proportional to the intensity of the reflected light 134 received by the array of photodetector units 116 over a period of time. The receipt of the trigger or synchronization signal by the signal generator 112 and the signal preparation and conditioning circuit 126 is used as an indication in time of when to start “recording” the electrical measurements generated by the array of photodetector units 116.

[0056] At the same time that the light source 108 emits the light pulses 132, the detection circuit 106 converts the light energy received at the photodetector units 116 into electrical measurements as follows. In accordance with the method described above, the signal generator 112 generates a mixing signal, which in this example has a flat spectrum, for example a time-varying mixing signal such as a PRBS mixing signal. It is beneficial, but not essential, to use a mixing signal with a flat spectrum. The length of the mixing signal can be N bits, the length depending on the frequency of the mixing signal and the observation distance of the device. For example, at a frequency of 100 MHz and a sequence length of 128 bits, an unambiguous distance of approximately 150 m can be achieved. A sequence length of 256 bits can yield an unambiguous distance of about 300 m. The phase selector 114 issues a control signal to the signal generator 112 to shift the phase of the PRBS mixing signal by ph bits (step 204), and the phase-shifted PRBS mixing signal is applied to the photonic mixer device 124 of the photodetector units 116 (step 206) in order to mix the photocurrent generated in response to the light 134 received by the photonic mixer 124 with the phase-shifted PRBS mixing signal. The photonic mixer 124 generates a photocurrent signal that is responsive to the light energy received thereby and the PRBS mixing signal applied to the modulation electrode 118 of the photonic mixer 124. The photocurrent signal is accumulated in the integrator 120.

[0057] The signal preparation and conditioning circuit 126 determines whether P optical pulses have been transmitted (step 208) in order to determine when the integrator 120 has accumulated photo-currents with respect to P consecutively received optical echo signals. If the number of generated photo-currents is insufficient due to an insufficient number of received reflected optical echo signals, the signal preparation and conditioning circuit 126 waits for the generation of P photo-currents. Thereby, the same variant of the phase-shifted mixed signal (i.e. the mixed signal with the same phase shift applied thereto) with respect to P consecutively received optical echo signals is applied to the photomixer 124 via the modulation electrode 118 of the photomixer 124. It is thus understood that in this example, a given stored electrical measurement is with respect to a number of reflected echo signals of light.

[0058] When the predetermined number (P) of optical pulses has been transmitted and the corresponding consecutive photo-currents with respect to the reflected echo signals have been generated, the signal preparation and conditioning circuit 126 reads out the integrated photo-currents constituting the electrical measurement and measures the value of the integrated photo-currents and stores the value of the electrical measurement (step 210).

[0059] In practice, the system 100 comprises a plurality of photodetector units 116. Thus, for each variant (in the phase-shifted variants) of the mixed signal applied to the plurality of photodetector units 116 while the plurality of photodetector units 116 generate their respective electrical measurements, the stored electrical measurements generated by the plurality of photodetector units 116 respectively for each variant are read out by the signal preparation and conditioning circuit 126 substantially in parallel, aggregated for each phase shift and stored. For example, for the first phase shift phl, the electrical measurements generated using the first phase shift phl and stored in all photodetector units 116 of the photodetector unit array are read out substantially in parallel, aggregated for the first phase shift phl and stored. Similarly, this process is repeated for the subsequent phase shifts phn. The aggregated electrical measurements, generated or recorded, for example, as a vector V corr [ph] are stored such as in a memory (not shown) corresponding to the currently applied phase shift ph. The phase selector 114 then increments the phase shift counter ph by one (step 212). In this respect, the phase of the mixed signal is serially shifted to achieve the variants of the phase-shifted mixed signal.

[0060] The phase selector 114 then determines (step 214) whether the phase shift counter ph is still smaller than the predetermined limit N corresponding to the length of the mixed signal. If the predetermined limit N has not been reached, the integrator 120 is cleared, any temporary storage elements used by the signal preparation and conditioning circuit 126 in the course of measuring the value stored in the integrator 120 are also cleared (step 216), and the above-described measurement steps (steps 202 to 212) are repeated and the new electrical measurements generated subsequently are appended to the electrical measurement vector V corr[ph], until all measurements for a complete cycle of phase shift are completed. Therefore, the photodetector unit 116 generates multiple stored electrical measurements over time and according to different variations of the phase-shifted mixed signal. As can be seen from the above steps, during the generation of stored electrical measurements, phase shift variations of the mixed signal are applied to the photon mixer 124 of the photodetector unit 116; these applications relate to continuous light pulses or groups of continuous light pulses.

[0061] However, if the cyclic phase shift counter ph has reached the predetermined limit N, then the electrical measurement vector V with respect to the complete cycle of the phase shift will be... corr [N] is considered as the complete output (step 218), and then the signal reconstruction unit 128 deconvolves the electrical measurements generated by the applied phase shift (step 220).

[0062] In this regard, in one embodiment ( Figure 3 , Figure 4 and Figure 5 In the signal reconstruction unit 128, the electrical measurement vector V is... corr [N] Perform FFT (step 300) and perform FFT (step 302) on the PRBS mixed signal 402 to generate a cross-correlation signal. The spectrum C(404) and the spectrum M(406) of the PRBS mixed signal, in this example, the cross-correlation signal It is an electrical measurement vector V corr [N]. Then, according to the above equation (3), the FFT of the PRBS mixed signal 406 is used to measure the electrical measurement vector V. corr The result of [N] is subjected to an inverse FFT (step 304), and then, for example, a low-pass Gaussian filter. This produces the reconstructed signal spectrum, such as the filtered spectrum 408 of the recovered echo waveform.

[0063] refer to Figure 6 Once inverse filtering has been performed, the time-domain echo waveform 410 is obtained by performing an IFFT (step 306) on the output of the inverse filtering process (i.e., the spectrum 408 of the recovered echo waveform). In this respect, the reconstructed signal spectrum 408 is converted to the time domain. For example, from... Figure 6 The comparison shows that the recovered time-domain echo waveform 410 tracks the original received reflected light signal 400 very well. The time-domain echo waveform 410 is the time-domain representation of multiple reflected light pulses incident on the photodetector unit 116.

[0064] In another embodiment ( Figure 7 , Figure 8 and Figure 9 In the previous example, the signal reconstruction unit 128 reconstructs the electrical measurement vector V. corr[N] (600) to generate the cross-correlation signal However, in this example, the mixed signal used is a simple periodic function mixed signal 602 (e.g. a square wave signal), and so the signal reconstruction unit 128 performs an FFT (step 502) on the mixed signal 602 to generate the spectrum M (606) of the periodic function mixed signal. The function K of equation (4) above defines a minimum of the spectrum M (606) of the mixed signal, as shown by the horizontal dashed line 605. The cross-correlation signal V corr [N] is then Wiener filtered (step 504).

[0065] Referring to Figure 10 Once Wiener filtered, the time domain echo waveform 608 is obtained by performing an IFFT (step 506) on the output of the Wiener filter. As can be seen from a comparison of Figure 10 The recovered time domain echo waveform 608 tracks the original received reflected light signal 600, as can be seen from a comparison of

[0066] Once the signal reconstruction unit 128 has recovered the time domain echo waveform 410, 608, the digital time domain echo waveform 410, 608 is passed to the microprocessor 130, which post-processes the echo waveform 410, 608, e.g. analyses the peaks of the digitised echo waveform 410, 608 to identify reflected light pulses and determine the distance to the reflecting cause, e.g. as described in the above-mentioned co-pending European patent application No. 18165668.7.

[0067] It will be appreciated by the skilled person that the above-described implementation is merely an example of the various implementations conceivable within the scope of the appended claims. Indeed, it will be appreciated that, for example, other mixed signals having a suitable spectral distribution can be used.

[0068] Although in the above-described example the electrical signal generated by the photon mixer 124 is derived from the number P of photocurrents generated with respect to P consecutive reflected light echo signals incident on the photodetector unit 116, it will be appreciated that this implementation is optional, and the stored electrical signal can be generated from a single photocurrent generated by the photon mixer 124 in response to a single reflected light echo signal.

[0069] It will be appreciated that the use of programmable elements described herein is purely exemplary, and the skilled person will appreciate that the programmable elements can be replaced with a "hardwired" implementation, e.g. using digital logic circuitry.

[0070] Although the spectrum of the mixed signal is generated in real-time or "on the fly" herein, it will be appreciated that the spectrum M of the mixed signal can be predetermined, stored, and provided, e.g., by retrieval from a memory, for subsequent use in the methods described herein.

[0071] In the above examples, the mixed signal is a PRBS or a periodic signal. It will be appreciated that the term periodic is taken in the sense that the period of the periodic signal is applied on a shot-by-shot basis, i.e., with respect to each emission of the light source 108. Indeed, this is an example of a mixed signal derived from a periodic signal. The signal also has the property that it can be shifted in time. This property is suitable for all types of mixed signals. It will thus be appreciated that any suitable signal comprising a repeatable time-varying portion, e.g., a periodic signal, can be used to provide the mixed signal, but a repeatable non-periodic signal or portion of a non-periodic signal can be used. In some embodiments, the mixed signal can be an analog signal.

[0072] It will be appreciated that references herein to "light" are intended as references to the optical range of the electromagnetic spectrum, e.g., between about 350 nm and about 2000 nm, such as between about 550 nm and about 1550 nm, or between about 600 nm and about 1000 nm, unless explicitly stated otherwise.

[0073] Use herein of the term particular functional means is to be understood as exemplary only and the skilled person will appreciate that such functionality can be provided in different functional means / or such functionality can be distributed over multiple functional means.

[0074] Alternative embodiments of the application can be implemented as a computer program product for use with a computer system, the computer program product, for example, being a series of computer instructions stored on a tangible data recording medium such as a diskette, CD-ROM, ROM, digital memory or fixed disk, or embodied in a computer data signal that is transmitted by a tangible medium or wireless medium, e.g., microwave or infrared. The series of computer instructions can constitute all or part of the functionality described above, and can also be stored in any volatile or non-volatile storage medium, such as a semiconductor, magnetic, optical or other memory device.

Claims

1. A method of generating a time-domain echo waveform, the method comprising: a pulsed electromagnetic radiation trigger source emitting a plurality of electromagnetic radiation pulses in response to a trigger signal; a plurality of electromagnetic radiation reflection echo signals irradiating an electromagnetic radiation detector unit, the electromagnetic radiation detector unit generating a plurality of electrical measurements in response to electromagnetic radiation incident on the electromagnetic radiation detector unit, the plurality of electrical measurements to be stored in the electromagnetic radiation detector unit; generating a time-varying mixing signal; applying a phase-shifted version of the mixing signal to the electromagnetic radiation detector unit while the plurality of electrical measurements are being generated in the electromagnetic radiation detector unit; a signal pre-processor reading out a plurality of stored electrical measurements from the electromagnetic radiation detector unit; a signal reconstruction unit generating a frequency spectrum of the plurality of stored electrical measurements and providing a frequency spectrum of the mixing signal; the signal reconstruction unit generating a reconstructed signal frequency spectrum by deconvolving the frequency spectrum of the plurality of stored electrical measurements using the frequency spectrum of the mixing signal; and the signal reconstruction unit generating the time-domain echo waveform by converting the reconstructed signal frequency spectrum to the time domain.

2. The method of claim 1, wherein the time-varying mixing signal is a pseudo-random binary sequence (PRBS) signal.

3. The method of claim 2, wherein deconvolving a frequency spectrum of a plurality of accumulated electrical measurements comprises inverse filtering the frequency spectrum of the plurality of stored electrical measurements with the frequency spectrum of the mixing signal.

4. The method of claim 1, wherein the time-varying mixing signal is a periodic signal.

5. The method of claim 4, wherein deconvolving the frequency spectrum of the plurality of stored electrical measurements comprises Wiener deconvolution of the frequency spectrum of the plurality of stored electrical measurements with the frequency spectrum of the mixing signal.

6. The method of any one of claims 1-5, wherein the phase-shifted versions of the mixing signal are generated by serially phase-shifting the mixing signal.

7. The method of any one of claims 1-5, wherein the electromagnetic radiation detector unit is a photonic mixer device.

8. The method of claim 7, wherein the photonic mixer device comprises a modulation electrode, the method further comprising: applying the phase-shifted versions of the mixing signal to the modulation electrode.

9. The method of any one of claims 1-5, further comprising: providing a time-varying mixing signal generator to apply the phase-shifted versions of the mixing signal to the electromagnetic radiation detector unit, respectively; generating the trigger signal; applying the trigger signal to the pulsed electromagnetic radiation trigger source; and applying the trigger signal to the time-varying mixing signal generator substantially simultaneously.

10. A method of measuring a distance to a reflecting object, comprising: detecting a reflection echo signal of electromagnetic radiation by analyzing the time-domain echo waveform generated using the method of generating the time-domain echo waveform of any one of the preceding claims, wherein the distance is determined based on the reflection echo signal of the electromagnetic radiation. ​ 11. The method of claim 10, further comprising: calculating the distance from a pulse of a plurality of pulses in the detected reflected echo signals of electromagnetic radiation for the pulse.

12. The method of any one of claims 10 or 11, wherein generating an electrical measurement value of the plurality of electrical measurement values comprises: a predetermined number of the plurality of electromagnetic radiation reflected echo signals are incident on the electromagnetic radiation detector unit consecutively and the electromagnetic radiation detector unit generates a corresponding number of individual electrical measurement values respectively in response to the predetermined number of the plurality of electromagnetic radiation reflected echo signals; applying a variation of the phase-shifted variations of the mixed signal to the electromagnetic radiation detector unit while the electromagnetic radiation detector unit generates the corresponding number of individual electrical measurement values; and accumulating the corresponding number of individual electrical measurement values.

13. The method of any one of claims 10 or 11, wherein an electrical measurement value of the plurality of electrical measurement values is with respect to a number of the plurality of electromagnetic radiation reflected echo signals.

14. The method of any one of claims 10 or 11, further comprising: providing a plurality of electromagnetic radiation detector units including the electromagnetic radiation detector unit; applying each variation of the phase-shifted variations of the mixed signal to the plurality of electromagnetic radiation detector units while each of the plurality of electromagnetic radiation detector units generates the plurality of electrical measurement values respectively for each variation of the phase-shifted variations of the mixed signal, the plurality of electrical measurement values to be stored in the electromagnetic radiation detector unit; the signal pre-processor reads out the stored electrical measurement values from the plurality of electromagnetic radiation detector units substantially in parallel respectively, each read out being for each variation of the phase-shifted variations of the mixed signal; the signal pre-processor aggregates the electrical measurement values read out from the plurality of electromagnetic radiation detector units for each parallel read out, thereby generating a plurality of aggregated stored electrical measurement values; the signal reconstruction unit generates a frequency spectrum with respect to the plurality of aggregated stored electrical measurement values and generates a frequency spectrum of the mixed signal; the signal reconstruction unit generates a reconstructed signal frequency spectrum by deconvolving the frequency spectrum of the plurality of aggregated stored electrical measurement values using the frequency spectrum of the mixed signal; and the signal reconstruction unit generates the time-domain echo waveform by the converting the reconstructed signal frequency spectrum to the time domain.

15. An electromagnetic radiation echo waveform generation system, comprising: a trigger source of pulsed electromagnetic radiation configured to emit a plurality of electromagnetic radiation pulses in response to a trigger signal; an electromagnetic radiation detector unit configured to generate a plurality of electrical measurement values in response to a plurality of electromagnetic radiation reflected echo signals irradiating the electromagnetic radiation detector unit, the plurality of electrical measurement values to be stored in the electromagnetic radiation detector unit; a time-varying mixed signal generator configured to apply phase-shifted variations of a mixed signal to the electromagnetic radiation detector unit while the electromagnetic radiation detector unit generates the plurality of electrical measurement values. and a signal pre-processor configured to read out the plurality of stored electrical measurements from the electromagnetic radiation detector unit; wherein a signal reconstruction unit is configured to generate a frequency spectrum with respect to the plurality of stored electrical measurements and to generate a frequency spectrum of the mixed signal; the signal reconstruction unit is configured to generate a reconstructed signal spectrum by deconvolution of the frequency spectrum of the plurality of stored electrical measurements using the frequency spectrum of the mixed signal; and the signal reconstruction unit is configured to generate a time domain echo waveform by converting the reconstructed signal spectrum to the time domain.

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