Apparatus and method for dual comb distance metrology using multi-photon detection

Abstract

An optical distance measuring or ranging device is described. The device comprises at least one optical pulse generator (30, 32; 80, 94; 120, 122; 180, 202; 300, 320) for generating a gating pulse train and a probe pulse train, the gating pulse train having a different repetition rate than the probe pulse train. The gating and probe pulses can be ultrashort laser pulses generated by different free-running mode-locked lasers. An optical probe arrangement is also provided for directing the probe pulse train to one or more objects (42, 44; 84, 86; 188, 190; 232, 234, 236; 306, 310) and for collecting return probe pulses returned from the one or more objects. The objects can include a target object and a reference object. The device comprises a multi-photon effect detector (58; 104; 140; 210; 324, 330) and is configured to direct both the gating pulse train and the return probe pulses to the multi-photon effect detector. The device can be used for industrial inspection, machine calibration, position measurement, etc.

Description

[0001] This invention relates to optical distance measurement, and more particularly to an improved apparatus and method for optical distance measurement using multiphoton effect detection.

[0002] Many techniques for optical distance measurement are known. The inherent trade-offs between accuracy / precision, ambiguity range, and update rate often lead to the optimization of such techniques to achieve either high precision or extended range capabilities. For example, carrier wavelength interferometry involves analyzing changes in the phase of an optical signal to determine distance. Precisely focused carrier wavelength interferometry techniques such as multiwavelength interferometry (MWI) and frequency-scanning interferometry (FSI) can achieve nanometer-scale resolution but require slow scans to measure distances longer than a few millimeters. Conversely, light detection and ranging (LIDAR) is a pulse-time-of-flight technique that records the arrival time difference of individual pulses to determine distance. While LIDAR can rapidly detect long ambiguities, its resolution is limited to a few millimeters due to the limited bandwidth of state-of-the-art photodetectors.

[0003] US2011 / 0285980 describes a coherent dual-comb distance metrology system capable of high-precision absolute distance measurements over long ambiguity ranges with a rapid update rate. As illustrated in Figure 1 (see also US2011 / 0285980), this system... Figure 3 Coherent dual-comb distance metrology employs two mode-locked lasers (also known as frequency combs) with nearly identical pulse repetition frequencies. The first frequency comb is the so-called probe comb, which operates at a first repetition rate (f... rep-probe ) generates an ultrashort pulse train 2. The second frequency comb is the so-called local oscillator (LO) comb, which operates at a slightly different repetition rate (f rep-LO An ultrashort pulse train 4 is generated. The ultrashort pulse train 2 generated by the probe comb is reflected from the target mirror 6 and also partially reflected from the surface 8 of the transmission reference optics 10. The target pulse 12 and the reference pulse 14 are generated by the reflection of the ultrashort pulse 2 generated by the probe comb and return from the target mirror 6 and the reference optics 10, respectively. The target pulse and the reference pulse are combined with the ultrashort pulse train 4 at the beam splitter 16 (i.e., generated by the LO comb) and transmitted to the detector 18.

[0004] As mentioned above, the repetition frequencies of the two frequency combs are slightly different (difference Δf). rep In one embodiment of US2011 / 0285980, the probe comb and LO comb have repetition rates of 100.021 MHz and 100.016 MHz, respectively. Therefore, the LO pulse is used to time-gated the pulses returning from the probe comb to create a series of interferograms. Specifically, detector 18 sees the difference in repetition rate (Δf)rep A series of repeating interferogram pairs are performed. The inset of Figure 1 shows the interferogram generated at detector 18 due to the interference of the probe pulse and the LO pulse. Therefore, the arrival time difference between the reference pulse and the target pulse in each interferogram pair provides a time-of-flight measurement of the distance between the reference optics 10 and the target mirror 6. The time-of-flight measurements can be repeated and averaged until the accuracy of the measurement drops below λ / 4, at which point interferometric phase analysis of the interferograms can be used to provide a higher resolution correction for the time-of-flight measurement. For example, the system of US2011 / 0285980 is shown to achieve an accuracy of 200 μm using time-of-flight information within an average of 200 μs. By combining the phase information from interferometric analysis, the accuracy is improved to 5 nm, with a total measurement time of 60 ms.

[0005] Therefore, coherent dual-comb distance metrology of the type described in US2011 / 0285980 has been found to successfully measure absolute distances of arbitrary lengths. This technique thus offers advantages over MWI, FSI, lidar, and other common alternatives because it can measure absolute distances with high accuracy, a long ambiguity range, and a fast update rate. The time gating of the probe pulse and the LO pulse also eliminates the risk of stray reflections that would cause systematic errors in other techniques.

[0006] Despite its various advantages over alternative optical metrology techniques, coherent dual-comb metrology of the type described in US2011 / 0285980 has not yet transitioned from laboratory research to industrial applications for several reasons. First, the accuracy of the time-of-flight portion of dual-comb measurements is limited by timing jitter from the combs (i.e., deviations in the rate at which pulses leave the comb cavities). Furthermore, once the measurement is transferred to interferometric analysis, the accuracy is also limited by phase noise from the combs. For this reason, it is considered necessary to lock the repetition frequency (f) of the two combs. rep ) and carrier envelope offset frequency (f CEO This reduces measurement uncertainty. However, a drawback is the need for multiple locking loops with several stable reference sources, making the entire system very complex and relatively expensive to manufacture. This is one of the main reasons why coherent dual-comb metrology systems have not been adopted in industrial metrology applications.

[0007] Several attempts have been made to reduce the complexity of coherent dual-comb metrology systems to allow their use in practical applications. The following is a brief overview of some examples of these variant systems.

[0008] Liu et al. (Optics Letters, 2011, Vol. 19, No. 19) describe a simplified system that uses a free-running comb and extracts time-of-flight information solely from interferograms. The data analysis performed to extract time-of-flight data from interferograms involves performing a Hilbert transform on the raw data to extract the envelope of each interferogram. A Gaussian or sech function is then fitted to each envelope to determine its center point. In such a system, the accuracy of the time-of-flight measurement is limited by the accuracy with which the center point of the interferogram can be determined. If the interferogram is unclear, perhaps because it contains too few interference fringes, the Hilbert transform fails and the calculated envelope of the interferogram is distorted, leading to inaccurate distance measurements. The system described by Liu et al. manages to achieve an accuracy of 2 μm within a measurement time of 140 μs, and on average, the accuracy improves to 200 nm within an acquisition time of 20 ms.

[0009] Zhang, H et al. (Optics Letters, 2014, 22(6), pp. 6597–6604) also demonstrated the use of optical cross-correlation. In this arrangement, the interferogram passes through a periodically polarized potassium titanyl phosphate (PPKTP) crystal to generate a second harmonic signal. This produces an intensity cross-correlation that is equivalent to the envelope of the interferogram. Although this technique has been shown to produce results similar to those described in US2011 / 0285980, the PPKTP crystal increases the complexity of the system and incurs additional costs.

[0010] Shi, H et al. (2015, CLEO: Science & Innovation, SF2L-3, Optical Society of America) demonstrated another technique involving the use of PPKTP crystals in balance detection, known as Balanced Optical Cross-Correlation (BCC). The two second harmonics generated in balance detection are collected to produce an S-shaped signal. The zero-crossing point with a steep central slope quantifies the time delay between the reference and target interferograms. Theoretically, this technique is attractive because the steep slope of the signal can facilitate the identification of zero-crossing points with higher accuracy than the identification of the center point of the interferogram envelope. However, the very precise optical alignment required for balance detection is highly undesirable for industrial applications.

[0011] Although dual-comb metering is considered a promising application for many uses, the various limitations mentioned above have hindered its use in industrial applications. Despite considerable efforts by numerous research teams over the years to overcome these limitations, these challenges remain.

[0012] According to a first aspect of the present invention, an optical distance measuring device is provided, comprising:

[0013] At least one optical pulse generator is provided for generating a gating pulse train and a probe pulse train, the gating pulse train having a different repetition rate than the probe pulse train.

[0014] An optical detection arrangement is provided for directing the detection pulse train to one or more objects and for collecting return detection pulses from the one or more objects.

[0015] The device is characterized in that it includes a multiphoton effect detector and is configured to direct both the gating pulse train and the return probe pulse to the multiphoton effect detector.

[0016] Therefore, a first aspect of the present invention provides an optical distance measuring or ranging device for measuring the distance to one or more objects. The optical distance measuring device can be used in a variety of different applications. For example, it can be used as a laser tracker to measure the position of one or more points on the surface of an industrial object, to calibrate coordinate measuring machines, as an encoder to measure the relative position of moving parts of a machine, or for large-scale length measurement, etc.

[0017] The apparatus includes at least one optical pulse generator that produces a gating pulse train and a probe pulse train. As explained below, the gating pulse and probe pulse are preferably ultrashort laser pulses. These ultrashort laser pulses can be generated by two separate mode-locked lasers; for example, a local oscillator mode-locked laser and a probe mode-locked laser, respectively. The gating pulse train is generated at a different repetition rate than the probe pulse train. Also explained in more detail below, pulse trains generated at different pulse frequencies (i.e., different repetition rates) will overlap (jump) each other in time at a frequency equal to the difference between the pulse frequencies. In this way, when the probe pulse train and the gating pulse train are combined, the gating pulse is used to gating the probe pulse.

[0018] The optical detection arrangement of the device guides a train of detection pulses generated by at least one optical pulse generator toward one or more objects. The train of detection pulses can be considered to form an optical detection beam, and the optical detection arrangement may include various optical components (lenses, mirrors, waveplates, optical filters, etc.) that guide or direct the detection beam toward one or more objects along a desired optical path. The optical detection arrangement is also configured to collect returned detection pulses returning (reflected) from one or more objects. The returned detection pulses may be reflected back from the object along the same optical path taken by the detection pulses incident on the object. Of course, the detection beam may be reflected or partially reflected from multiple different objects. In a preferred embodiment described below, the optical detection arrangement may be arranged such that the detection pulses return from both a reference object and a target object. In such an example, the returned detection pulses may include a reference pulse train (i.e., returned from a reference object) and a target pulse train (i.e., returned from a target object).

[0019] Both the gating pulse train and the return probe pulse are directed to a multiphoton effect detector. The so-called multiphoton effect is a transient effect caused by two or more photons arriving together (e.g., no more than one femtosecond apart) at the detector. Two or more photons are absorbed, and the detector responds to the combined photon energy. In other words, two or more photons are absorbed together as if they were a single photon. For multiphoton effect detectors comprising semiconductor materials, the combined energy of the multiple photons absorbed together is sufficient to excite a single photoelectron to cross the band gap, thereby generating a detectable photocurrent. The multiphoton absorption effect response exhibited by such a multiphoton effect detector is typically strongly nonlinear. Therefore, the peak value in the intensity of the output electrical signal generated when a multiphoton absorption event occurs (i.e., due to the simultaneous arrival of the gating pulse and the return probe pulse) is easily distinguishable from background levels (e.g., due to random two-photon events, electrical noise, etc.).

[0020] Therefore, the multiphoton effect detector detects the (temporally) coincident arrival of the probe pulse and the gating pulse. As explained above, the frequency at which the probe pulse and the gating pulse arrive simultaneously at the multiphoton effect detector is equal to the difference in repetition frequencies of the probe pulse train and the gating pulse train. The multiphoton effect detector allows for the determination of the time of such events, from which distance (time-of-flight) information can be obtained. For example, gating of return pulses from a target object (i.e., the target pulse) and a reference object (i.e., the reference pulse) enables the measurement of the distance between the reference object and the target object.

[0021] Importantly, the use of the multiphoton effect according to the invention (i.e., the simultaneous arrival of both the detection gating pulse and the return probe pulse) eliminates the requirement for optical interference between the return probe pulse and the gating pulse in prior art systems. This is because the multiphoton absorption effect is an energy effect, not an optical interference effect. Therefore, the use of the multiphoton effect detector removes the fundamental requirement associated with prior art dual-comb systems (e.g., as described in US2011 / 0285980), namely, that the gating photon and the probe photon must optically interfere at the detector.

[0022] Therefore, this invention has advantages over prior art (interference-based) systems (such as those described in US2011 / 0285980) in that it does not require controlling the phase of the electric field within each pulse to ensure accurate extraction of the envelope of the resulting interferogram. This means that the need for phase control within the pulse generator can be avoided, and therefore a free-running mode-locked laser can be used to generate the gating and probe pulses. The use of a multiphoton effect detector also allows for optical amplification of the returned probe pulse, because, unlike interferometric systems, the quality of the optical wavefront arriving at the multiphoton effect detector has minimal impact on the resolution of the multiphoton absorption detection process. Furthermore, the use of a multiphoton effect detector removes the requirement to ensure that the gating and probe pulses have substantially overlapping wavelengths (which is necessary to obtain optical interference).

[0023] Therefore, the present invention provides a distance measuring device that is simpler, more flexible, and cheaper to manufacture than the dual-comb distance measuring system of the type previously described in US2011 / 0285980. In particular, the distance measuring device of the present invention has been found to be very suitable for industrial metrology applications.

[0024] It should be noted that multiphoton effect detection has previously been proposed in various technical fields to provide autocorrelators for interferometric measurements. US6195167 describes an autocorrelator for measuring the duration of an ultrashort laser pulse using a two-photon effect detector. The autocorrelator is arranged to measure pairs of sub-pulses with relative delays obtained from the parent pulse using an interferometer. Such autocorrelator devices have previously been used to characterize ultrashort laser pulses (e.g., to measure pulse shape, pulse duration, etc.).

[0025] Advantageously, the multiphoton effect detector has a bandgap with photon energies greater than those of a single photon in the gating and probe pulses. Photon energy, the energy carried by a single photon, as explained in more detail below, is proportional to the photon's electromagnetic frequency and, therefore, inversely proportional to its wavelength. Thus, the wavelengths of the gating and probe pulses are preferably long enough (i.e., the frequencies of such pulses are low enough) that the energy of a single photon is less than the detector's bandgap. This means that the multiphoton effect detector is essentially insensitive to single photons in the gating and probe pulse trains (i.e., because such photons do not have sufficient energy to excite photoelectrons across the bandgap). However, the detector does detect the (temporally) coincident arrival of two or more photons with combined energies greater than the bandgap.

[0026] Advantageously, the multiphoton effect detector includes a two-photon detector. In such an arrangement, the combined energy of the gated pulse photon (i.e., the photon of the gated pulse) and the probed pulse photon (i.e., the photon of the probed pulse) is preferably greater than the band gap of the two-photon effect detector. In other words, the energy imparted when the two-photon effect detector absorbs both the probed photon and the gated photon together is sufficient to sufficiently excite photoelectrons, causing them to cross the band gap. As explained below, the two-photon absorption effect is a strongly nonlinear (quadratic) effect.

[0027] While two-photon effect detection is advantageous for simplicity, it should be noted that multi-photon effect detection can be implemented, where three or more photons arriving together at the multi-photon effect detector have a combined energy greater than the detector's bandgap. Taking three-photon detection as an example, the bandgap of the multi-photon effect detector can be chosen to be greater than the combined energy of two photons (i.e., the combined energy of two photons arriving together is insufficient to excite photoelectrons across the bandgap), but less than the combined energy of three photons. Similar considerations will apply to higher-order multi-photon detection (e.g., four-photon detection, etc.).

[0028] Multiphoton effect detectors can be semiconductor devices. For example, they can be light-emitting diodes (LEDs), photodiodes, avalanche photodiodes, or laser diodes. Multiphoton effect detectors can include semiconductor materials. Advantageously, the semiconductor material has a band gap with an energy higher than that of a single photon from the gating and probe pulses. For example, semiconductor materials can include silicon, GaAsP, AlGaAs, InGaAs, Ge, GaN, InGaN, InGaAsP, GaP, InGaP, GaAs, or PbSe, all of which allow operation as photovoltaic detectors. Advantageously, multiphoton effect detectors include silicon. Silicon is particularly preferred when using gating and probe pulses with mid-infrared optical wavelengths. For example, when the wavelengths of the gating and probe pulses are greater than 1110 nm and less than 2200 nm, two-photon effect detection can be performed using a silicon-based detector. Silicon-based detectors are advantageous because of their relatively low cost and ease of use, including the ability to operate at ambient temperatures. As an alternative to using detectors comprising semiconductor materials, multiphoton detection can be implemented using fluorescent dyes (e.g., as used in multiphoton microscopy). Such devices can also be configured to have a suitable bandgap (which can also be called an energy gap).

[0029] Therefore, the device includes a multiphoton effect detector; that is, a detector arranged within the device to perform multiphoton effect detection. Multiple additional components can be provided as part of the device to provide or enhance the multiphoton effect. As explained above, the bandgap of the detector is preferably arranged to suit the photon energy of the gating / probe pulse. Elements may also be present within the device to focus the probe pulse and the gating pulse at the detector. For example, the probe pulse and the gating pulse can be focused to at least partially overlap on the detector. Advantageously, the probe pulse and the gating pulse can be focused (e.g., closely focused) at substantially the same point on the detector. In a preferred embodiment, the returned probe pulse and the gating pulse can be combined (e.g., using a beam splitter), and the combined beam of the returned probe pulse and the gating pulse is delivered to the multiphoton effect detector along a common path. Alternatively, the gating pulse and the returned probe pulse can be delivered to the detector along separate optical paths.

[0030] A multiphoton effect detector may include a single detector element. In other words, a multiphoton effect detector may include a single pixel or a single active region. Alternatively, a multiphoton effect detector may include multiple detector elements. These multiple detector elements may be spatially separated from each other, for example, in a linear or two-dimensional array. In such an example, a gating pulse train may be divided (e.g., using a beam splitter) and directed to each of the detector elements. Return probe pulses from different objects or from different points on the same object may then be directed to different detector elements. In this example, an electrical signal may be output for each detector element. Multiple distance measurements can then be performed simultaneously, as described in more detail below.

[0031] Advantageously, the apparatus includes an analyzer for analyzing the output of the multiphoton effect detector. The analyzer can analyze the relationship between the output of the multiphoton effect detector and time. The analyzer may include a digitizer (e.g., an analog-to-digital converter (ADC)) to digitize the electrical signal output by the multiphoton effect detector. The analyzer can fit the relationship between signal strength and time as a function. For example, a Gaussian function can be fitted to the signal strength data. The arrival times of the gating pulse and the return probe pulse at the detector can then be determined based on the peak values ​​in the digitized electrical signal. This, in turn, allows for time-of-flight distance measurements. The analyzer may include a processor for performing signal processing and / or analysis. If the multiphoton effect detector comprises multiple detector elements, signals from each detector element can be analyzed in this manner.

[0032] Conveniently, the apparatus includes a time-of-flight monitor. The time-of-flight monitor may include at least one peak detector. The time-of-flight monitor may include at least one timer. Thus, the at least one peak detector can be configured to detect peaks in the output of a multiphoton effect detector. The at least one timer can be configured to measure the time between detected peaks. The time-of-flight monitor may include a processor that performs peak detection and timing measurements. If the returned probe pulses are from both a reference object and a target object, the time between the detected peaks can be established, correlated with the difference in the optical path lengths traveled by the probe pulses returning from the reference object and the target object. This allows the distance between the reference object and the target object to be determined. This type of time-of-flight monitoring is computationally inexpensive and therefore potentially faster than collecting and subsequently analyzing the relationship between signal strength data and time.

[0033] An optional time-of-flight monitor may include at least two timers. If probe pulses return from both the reference and target objects, a first timer may measure a first time interval between the peaks returned from the reference and target objects, while a second timer may measure a second time interval between the peaks returned from the target and reference objects. The time interval between the peaks returned by the reference object can also be measured. This can be done by adding the time intervals measured by the first and second timers. The time interval between the peaks returned by the reference object can be used as a timing reference signal, for example, to compensate for any changes in the repetition rate difference between the probe and the local oscillator comb. Using such a timing reference signal can reduce measurement uncertainty.

[0034] At least one optical pulse generator may include multiple optical pulse generators. Each of the multiple optical pulse generators can generate an ultrashort laser pulse train. Conveniently, at least one optical pulse generator includes a probe mode-locked laser for generating a probe pulse train and a local oscillator (LO) mode-locked laser for generating a gating pulse train. In other words, two separate mode-locked lasers (such mode-locked lasers are also called frequency combs) can provide probe pulses and gating pulses. The probe pulse train and gating pulse train are preferably ultrashort laser pulses. The duration of such ultrashort laser pulses is typically on the order of picoseconds or femtoseconds. For example, an ultrashort pulse can have a duration of less than 1 picosecond. Alternatively, the probe pulse train and gating pulse train can be obtained from a common laser cavity (e.g., a single optical pulse generator can be provided to generate both the probe pulse and the gating pulse).

[0035] Advantageously, both the probe mode-locked laser and the local oscillator mode-locked laser are so-called "free-running" mode-locked lasers. In other words, the phase of the electric field within the envelope of each pulse emitted by the laser can vary in an unknown way (i.e., there is no phase control loop). This is acceptable because the multiphoton effect detector does not depend on the generation of the interferogram and is therefore essentially unaffected by any changes in the carrier envelope phase shift. The ability to use separate optical pulse generators (e.g., without phase control) to generate both probe and gating pulses is a significant advantage compared to prior art systems.

[0036] As outlined above, multiphoton effect detection does not require any optical interference between the return pulse and the gate pulse at the detector. In fact, optical interference between the return probe pulse and the gate pulse can degrade the performance of the device. If optical interference between the gate pulse and the return pulse is allowed, an interferogram will be collected at the detector; however, if they are combined without interference, the detector only collects the envelope. Therefore, it is preferable that the pulses overlap temporally at the detector with minimal or no optical interference. Consequently, the optical configuration of the device is preferably arranged to substantially prevent optical interference between the gate pulse and the probe pulse at the multiphoton effect detector. This allows direct detection of the intensity envelope without the drawbacks associated with generating interference patterns.

[0037] The apparatus may include various optical components (lenses, waveplates, beam splitters, mirrors, etc.) that guide various gating and probe pulses from at least one optical pulse generator to a multiphoton effect detector. These optical components may be arranged to prevent any substantial optical interference between the gating and probe pulses as they arrive at the multiphoton effect detector. Advantageously, the gating pulse has a different polarization state than the probe pulse upon arrival at the multiphoton effect detector. In other words, the polarization states of the gating and probe pulses can be selected to prevent or minimize optical interference. For example, the gating and probe pulses may be configured to have substantially orthogonal linear polarizations upon arrival at the detector. Alternatively, the gating and probe pulses may have different circular polarizations (e.g., left-handed and right-handed circular polarizations) upon arrival at the detector. Optical interference effects can also be reduced or minimized by appropriately selecting the optical wavelengths of the gating and probe pulses. For example, if the optical bands of the gating and probe pulses are different (e.g., do not overlap), there will be no optical interference between these pulses. It will also be possible to control the incident angle of the gating pulse and the return probe pulse on the multiphoton effect detector to minimize optical interference.

[0038] Probe pulses are directed to one or more objects and return from one or more objects. The one or more objects may include at least one reference object. Advantageously, a single reference object may be provided. Such a reference object may be provided as part of the optical arrangement of the apparatus. For example, the reference object may be an internal optical component of the apparatus. The reference object may be positioned in a separate reference arm of the apparatus. Alternatively, the reference object may include a partially reflective component through which any unreflected light is transmitted to another (e.g., a target) object. The reference object may include a mirror. The reference object may include partially reflective (and therefore partially transmissive) components, such as a glass wedge. The reference object generates returning probe pulses, which are referred to herein as reference pulses (i.e., because they return from the reference object). The probe pulses may be incident on the reference object along the same path as the (returning) reference pulses. The position of the reference object relative to the rest of the apparatus is preferably constant (i.e., it provides a constant reference position or reference point).

[0039] One or more objects may include one or more target objects. Target objects may have a distance unknown to the rest of the apparatus. Therefore, one or more objects may include one or more remotely located target objects. For example, (multiple) target objects may be located outside the apparatus. Target objects may include the surface of the object to be measured. For example, the surface of an industrial part may be measured directly. Alternatively, a target object may be attached to another object. For example, a target reflector or retroreflector may be attached to another item to be measured (i.e., indirect measurement). Each target object generates a returning probe pulse, which is referred to herein as target pulses (i.e., because they are returned from the target object). It should also be noted that although a continuous series of such target pulses is typically returned by the target object, not all probe pulses directed to the object will return (e.g., some may miss the target entirely, or some may reflect in directions that do not allow them to be captured). The target object may also move relative to the apparatus, and therefore the target pulses can provide a snapshot of the position. Measuring the change in distance over time can also be used to measure the velocity of the target relative to the apparatus.

[0040] In a preferred embodiment, one or more objects include a reference object and one or more remotely located target objects. In this embodiment, the returned probe pulses include reference pulses returned from the reference object and target pulses returned from the one or more remotely located target objects. The arrival time difference between the target pulse and the associated reference pulse is related to the path length difference between the pulses returned from the target surface and the reference surface. If the reference object has a constant position, the distance to the target objects(s) can be determined based on the arrival time difference between the reference pulse and the target pulse.

[0041] The apparatus can be arranged to transmit probe pulses to a target object via free space (e.g., air). Alternatively, the probe pulses can be transmitted to the object via a liquid (e.g., water). The target object may have a rough surface that disrupts the optical waveform of the probe pulse (i.e., causing the target pulses collected by the apparatus to have a distorted or disrupted wavefront). Similarly, the medium through which the probe pulses travel to and from the target object may disrupt the optical wavefront of the probe pulses. For example, such interference may be caused by passing the probe pulses through turbulent air. In some cases, such as those listed above, the returning probe pulses may have low intensity (e.g., a high level of probe beam attenuation may be present). Optical amplification of the target pulses after they return from the target but before they reach the multiphoton effect detector can also improve sensitivity. Amplification of the reference pulse can also be provided. For example, an optical fiber amplifier can be used. Such amplification process may also degrade the quality of the optical wavefront. For prior art interferogram-based arrangements, such distortion of the optical wavefront would severely affect the quality of the obtained optical interference. However, when using multiphoton effect detection according to the invention, such effects have a much lower impact because it is not based on optical interference.

[0042] As outlined above, an optical detection arrangement can direct a detection beam (i.e., a train of detection pulses) to one or more objects. This can include directing the detection pulses to a single target on a single object. The apparatus can also be configured to measure multiple targets; such targets are on different objects or at different points on the same object. Therefore, the optical detection arrangement can include a beam splitter that divides an input detection beam comprising a train of detection pulses into multiple output detection beams. The term "splitting" refers to the optical intensity of the initial detection beam being divided into two or more output detection beams. Thus, each of the output detection beams contains a complete train of detection pulses, although its intensity is lower than that of the input detection beam. Each output detection beam can then be directed to one of the multiple targets. The optical detection arrangement is preferably also configured to collect return detection pulses from each of the multiple targets.

[0043] In the multi-target example above, the probe pulses returning from each target can be combined into a single beam, which is then directed to the same point on the multiphoton effect detector as the gating pulse. In such an example, the peak in the detected signal will correspond to the return probe pulse from each of the different targets. If the targets are at different distances, the distance to each target can be calculated individually.

[0044] Alternatively, a multiphoton effect detector may include multiple active elements or detector elements (e.g., pixels or regions) as described above. In such an example, the probe pulses returning from each target can be individually directed to different detector elements among the multiple detector elements. The multiple multiphoton effect detector elements may be provided by a multi-pixel sensor array formed on a single substrate, or by multiple physically separated multiphoton effect detector elements (e.g., photodiodes spatially separated from each other (e.g., in different arms of a device)). In such an example, the optical detection arrangement may also include an additional beamsplitter that splits the input local oscillator beam, comprising a gating pulse train, into multiple output local oscillator beams. Each of the multiple detector elements can then receive both the output local oscillator beam and the returning probe pulse from one of the multiple targets. Multiphoton effect detection at each active element is then correlated with the distance to the corresponding target. In other words, the peak value in the signal from each active element makes it possible to measure the distance to a single target. Of course, a reference pulse train can also be directed to all, some, or one detector element to also provide a reference peak (e.g., to aid in time-of-flight calculations of the distance). It should be noted that a reference peak generated by an active element can be used as a timing reference for a target peak detected by other detector elements. In other words, time-of-flight distance measurement can be performed using a reference peak detected by an active element (i.e., indicating the arrival time of a reference pulse) and a target peak detected by different active elements (i.e., indicating the arrival time of a target pulse).

[0045] The ability to simultaneously measure distances to multiple targets offers numerous advantages. For example, the distances to multiple independent objects can be measured simultaneously. If multiple target points on a single object are measured simultaneously, more information about the object's orientation (e.g., measuring pitch, roll, or yaw) or shape can also be established. Multiple points on an object can also be measured, or alternatively, in series. For example, a probe beam (i.e., a probe pulse train) can be scanned (e.g., rasterized) on the object's surface to measure multiple points on that object. In this way, a three-dimensional (3D) image of the object can be created.

[0046] Advantageously, the optical distance measuring device is configured for mounting on a coordinate positioning device such as a coordinate measuring machine (CMM). The CMM may include an articulated robotic arm; the optical distance measuring device can then be used as a distance measuring component of a laser tracker. The CMM may be a bridge-type CMM, wherein the sleeve is movable relative to a base on which the object to be inspected is placed. The object can be secured to the base of the CMM by a fixing device. The optical distance measuring device may have an optical head (e.g., including an optical detection arrangement) for emitting a probe pulse train and / or collecting return pulses. The optical head can be attached to the sleeve of the CMM. The optical head can be attached to the sleeve of the CMM via a dual-axis rotating probe (such as the REVO (registered trademark) articulated probe sold by Renishaw PIC, Wotton Under Edge, Gloucestershire, UK). At least one optical pulse generator of the device may be located near the CMM and connected to the optical head via one or more optical fibers.

[0047] Conveniently, optical distance measuring devices can be used to calibrate machines, such as coordinate positioning devices. Coordinate positioning devices can include CMMs (e.g., bridge-type CMMs, robotic arms, etc.) or machine tools. Therefore, the device can be temporarily mounted to the machine to perform the calibration process. In such an example, one or more retroreflectors or mirrors can be mounted to a movable part of the machine (e.g., multiple mirrors can be mounted to the sleeves or arms of a CMM). As the movable part of the machine moves around in space, the optical distance measuring device can then measure the position of the mirrors. The high-accuracy position measurements collected by the optical distance measuring device can then be used to correct or map errors in position measurements performed by the machine's internal measuring equipment (e.g., encoders). After calibration, the device can be removed from the machine.

[0048] Alternatively, the optical distance measuring device of the present invention can be incorporated into a machine. In other words, the optical distance measuring device can be integrated into the machine as a position encoder or the like. This allows for the measurement of the relative positions of different parts of the machine (e.g., as part of a machine position control loop). The advantage of the optical distance measuring device is that it can measure relative positions with no energy loss without the need to establish a reference position, and is therefore particularly suitable for larger machines.

[0049] Many other applications of the optical distance measuring device of this invention are also possible. For example, the optical distance measuring device can be used in large-scale length measurement applications, such as: large manufacturing and assembly machines, as a motion control feedback element; assembly of large production facilities, including jig and fixture alignment; civil engineering, for measuring new buildings and monitoring the movement of existing buildings during work on adjacent ground, etc.; asset management of large structures, including docks, bridges, dams, and dikes; profiling surveys; capturing and modeling historical / archaeological sites and artifacts for research, education, and virtual tourism; and as feedback for large machines and instruments to allow control of geometry, such as large optical telescopes. The device can also be used for condition monitoring of large safety-critical structures, and as a sensor for terrain / environment mapping for autonomous robots, vehicles, space (such as satellite tracking), and defense applications.

[0050] It should be noted that the optical distance measuring device of the present invention can be used in conjunction with other optical distance measuring devices. For example, a conventional coaxial lidar arrangement can be used in parallel with the device of the present invention. A conventional lidar arrangement can provide coarse measurements (e.g., within one or two meters), while the device of the present invention provides finer (i.e., much higher resolution) measurements. Therefore, coarse measurements can be used to extend the ambiguity range. For example, the optical distance measuring device of the present invention can use a local oscillator repetition rate of approximately 300 MHz, which will impart a ranging ambiguity of approximately 1 m. A standard lidar can then be used to determine the target increment as 1 m. Such a standard lidar system would require a timing resolution of more than 3 ns to achieve such resolution, which is very simple with readily available lidar technology.

[0051] According to a second aspect of the present invention, a method for optical distance measurement is provided, the method comprising the following steps:

[0052] (i) Generate a probe pulse train

[0053] (ii) Generating a gating pulse train, the gating pulse train being generated at a different repetition rate than the probe pulse train.

[0054] (iii) Direct the probe pulse train toward one or more objects, and collect the returned probe pulses from the one or more objects.

[0055] (iv) The gating pulse train and the return probe pulse are directed to a detector, wherein the detector is configured to perform multiphoton effect detection. The method may also include any of the steps mentioned above in the context of the relevant apparatus.

[0056] According to a further aspect of the invention, a multiphoton effect detection apparatus is provided, comprising a multiphoton effect detector and a timestamp analyzer. The timestamp analyzer includes at least one peak detector and at least one timer, the at least one peak detector being configured to detect peaks in the output of the multiphoton effect detector, and the at least one timer being configured to measure the occurrence time of the detected peaks. The time between the detected peaks can be measured to establish a time-of-flight measurement of a distance. The analyzer may include a processor providing multiphoton effect detection and timestamp analysis. Other uses of the apparatus are also possible.

[0057] According to a further aspect of the invention, there is a method for analyzing the output of a multiphoton effect detector, comprising the steps of: detecting intensity peaks in the output of the multiphoton effect detector caused by multiphoton absorption events; and measuring the time of occurrence of such detected intensity peaks. The time between detected peaks can be measured. In this way, time-of-flight measurements of distances can be performed.

[0058] According to a further aspect of the invention, a coordinate positioning device is provided, the device comprising an optical measurement probe for inspecting an object, wherein the optical measurement probe includes at least one frequency comb and a multiphoton effect detector. The optical measurement probe may include a distance measuring device according to a first aspect of the invention.

[0059] According to a further aspect of the invention, a calibration apparatus is provided for calibrating coordinate positioning devices (e.g., machine tools, coordinate measuring machines, robots, etc.), the calibration apparatus comprising at least one frequency comb and a multiphoton effect detector. The calibration apparatus may include a distance measuring device according to the first aspect of the invention.

[0060] According to a further aspect of the invention, a position encoder is provided for measuring the relative distance between two movable parts of a machine (e.g., a machine tool, a coordinate measuring machine, a robot, etc.), the position encoder comprising at least one frequency comb and a multiphoton effect detector. The position encoder may include a distance measuring device according to the first aspect of the invention.

[0061] The invention will now be described by way of example only, with reference to the accompanying drawings, in which;

[0062] Figure 1 illustrates a prior art dual-comb distance measuring device as described in US2011 / 0285980.

[0063] Figure 2 The distance measurement device of the present invention, including a multiphoton effect detector, is shown.

[0064] Figure 3 It shows Figure 2A variant of the device, which includes a separate reference surface arm,

[0065] Figure 4 The reference pulse and target pulse present in the two-photon signal are shown.

[0066] Figure 5 It shows Figure 4 A pair of target pulses and a reference pulse, wherein the inset shows the target pulses in more detail.

[0067] Figure 6 This illustrates the method for obtaining two-photon and interference-based methods. Figure 2 The distance measuring device of the measurement results of the person,

[0068] Figure 7a Two-photon pulses were shown and Figure 7b The interferogram signal is shown.

[0069] Figure 8 The diagram illustrates how variations in the interference pattern within an interferogram can lead to uncertainty in determining the envelope center.

[0070] Figure 9 The effect of pulse repetition rate on interferometric measurements and two-photon-based measurements is shown.

[0071] Figure 10 A distance measuring device including a return signal is shown.

[0072] Figure 11 A distance measuring device for simultaneously measuring multiple targets is shown, and

[0073] Figure 12 A distance measurement device with a microcontroller for timing the occurrence of two-photon events is shown.

[0074] Figure 13 These are experimental data showing the relationship between Allan bias and averaging time in measurements performed using the apparatus of the present invention.

[0075] Figure 14 shows the distance and residual error measured using the device of the present invention.

[0076] Figure 15 A further embodiment of the distance measuring device of the present invention is shown.

[0077] Figure 16 A further embodiment of the distance measuring device of the present invention is shown.

[0078] Figure 17 It shows Figure 16 A variant of the arrangement, which includes two lidar timers,

[0079] Figure 18 It shows the use of Figure 17 The timing information extracted from the two lidar timers,

[0080] Figure 19 It shows Figure 17 A variant of the arrangement in which the timing signal is extracted from the lidar timer, and

[0081] Figure 20 It shows Figure 19 A variant of the arrangement in which a high-speed photodiode is used to extract a timing reference.

[0082] refer to Figure 2 The diagram illustrates the distance measurement device of the present invention. The device includes a probe comb 30 and a local oscillator (LO) comb 32. Both the probe comb 30 and the LO comb 32 are SESAM mode-locked erbium fiber lasers. The outputs of these lasers can be considered in the time domain as 1 / f rep (where f) rep The laser output consists of ultrashort pulse trains separated by the repetition rate of the laser, or frequency combs that can be viewed as regularly spaced intervals in the frequency domain. In this embodiment, the probe comb 30 has a repetition rate of 78.87 MHz, and the LO comb 32 has a repetition rate of 78.874 MHz. The laser output has a wavelength of 1557 nm, is optically amplified to 20 mW (from 3 mW), and the resulting pulses have unknown polarization.

[0083] The probe pulse train from probe comb 30 has an unknown polarization and therefore passes through the first quarter-wave plate 34 and the first half-wave plate 36 to provide polarization control. Specifically, the probe pulses leave the first half-wave plate 36 with p-polarization, such that they are transmitted through the first polarization beam splitter prism (PBS) 38. The probe pulse train is then reflected from the reference (back) surface 40 of the glass wedge 42 and also (partially) from the surface of the target mirror 44. This produces reflected pulse trains from the glass wedge 42 and from the target mirror 44; these returning pulses can be referred to as the reference pulse and the target pulse, respectively. The optical path is configured such that the probe pulses pass through the second quarter-wave plate 46 in their path to the reference and target surfaces and in their return journey to the polarization beam splitter prism (PBS) 38. This double pass of the probe pulses through the second quarter-wave plate 46 transforms the pulses into s-polarization, such that they are reflected at the PBS 38 toward the second polarization beam splitter prism (PBS) 48.

[0084] LO comb 32 generates a train of gating pulses with unknown polarization. The gating pulse from LO comb 2 passes through a third quarter-wave plate 50 and a second half-wave plate 52, which are arranged to impart p-polarization to the gating pulse when it reaches the second polarizing beam splitter prism (PBS) 48. Therefore, it can be seen that the reflected (probe) pulse meets the gating pulse at the second PBS 11 with orthogonal linear polarization. The use of orthogonal polarization prevents any interference between the probe pulse and the gating pulse.

[0085] The probe and gating pulses pass through a 1300 nm optical long-pass (high-pass) filter 54 to block any unabsorbed pump light from the comb and any stray light that might enter the instrument through the probe light channel sensitive to the photodiode 58. The probe and gating pulses are then focused by a lens 56 onto a silicon photodiode 58. The photodiode 58 is chosen to have a bandgap greater than hc / λ and less than 2hc / λ, where h is Planck's constant, c is the speed of light, and λ is the wavelength of the comb. In this example, the bandgap of the silicon photodiode 58 is 1.11 eV. Therefore, the detector bandgap is greater than the photon energy of the laser radiation generated by the frequency comb, meaning the detector is insensitive to a single photon generated by the frequency comb. However, if the probe pulse focused onto the photodiode 58 coincides in time with the gating pulse, the probe pulse will cause a peak in the two-photon absorption signal. Specifically, a detected signal will be generated that is proportional to I(t)I(t-τ), where τ is the instantaneous delay between the two pulses (i.e., the delay between the probe pulse and the gating pulse). The two-photon absorption effect has a quadratic response to the input power, which means that when the gating pulse (i.e., the photon from the LO comb) coincides with the probe pulse (i.e., the photon from the probe comb) in time to a sufficient degree, the photodiode will generate a detectable signal.

[0086] The signal detected by photodiode 58 passes through electronic low-pass filter 60 before being displayed on digitizer 62 or oscilloscope. It should be noted that the low-pass filter can alternatively be implemented in software or even mechanically. Low-pass filter 60 is selected with a cutoff frequency of approximately half the repetition rate of the probe comb, such that the pulse repetition frequency is filtered out. This can also be achieved using an electronic bandpass filter at an appropriate frequency. In this example, a silicon (Si) photodiode is used to collect the two-photon signal; however, as discussed, many alternatives exist. Any optical sensor device with a bandgap greater than but less than twice the energy of a single photon can be used for two-photon detection. For example, photodiodes, avalanche photodiodes, light-emitting diodes (LEDs), reverse-drive laser diodes, photocathodes, or photomultiplier tubes can be used.

[0087] It is important to reiterate that the essentially simultaneous arrival of the two-photon effect used to detect the probe and gating pulses is not an interference-based effect. The two-photon absorption effect is polarization-independent and phase-matched. The combination of laser pulses with opposite polarizations means their phases cannot interact, therefore there is no interference, and only the envelope of the resulting intensity distribution is detected. This is important because the detected signal is then offset from the carrier envelope of the comb by a frequency (f...). CEO It is not sensitive to changes in ).

[0088] refer to Figure 2 The described apparatus is merely an example of how the principle of nonlinear two-photon detection can be implemented in a dual-comb distance measurement device. Several variations of the apparatus that can be used to implement the same principle will now be described.

[0089] refer to Figure 3 An example of an alternative optical arrangement is described. A probe comb 80 generates an ultrashort train of probe pulses, which are split into two paths by a (non-polarized) beam splitter 82. Probe pulses guided along the first path are reflected from a fixed-position reference mirror 84 and guide the reference pulse train (i.e., the probe pulses reflected from the reference mirror 84) back to the beam splitter 82. Probe pulses guided along the second path are reflected from the target surface of a movable mirror 86 and guide the target pulse train (i.e., the probe pulses reflected from the movable mirror 86) back to the beam splitter 82. The target pulse train and the reference (probe) pulse train are recombined at the beam splitter 82 before passing through a quarter-wave plate 88 and a half-wave plate 90, which are arranged to impart polarization to the returning probe pulses when they arrive at the polarized beam splitter 92.

[0090] LO comb 94 generates a train of ultrashort gating pulses that pass through another quarter-wave plate 96 and a half-wave plate 98, which are arranged to impart p-polarization to the LO pulses. This arrangement is configured such that the (s-polarized) probe pulse and the (p-polarized) gating pulse meet at polarization beam splitter prism 92 with opposite polarizations. (See reference...) Figure 2In the outlined example, this prevents any substantial optical interference between the gating pulse and the probe pulse. The combined probe and LO beams are guided from the beam splitter prism 92 to the long-pass filter 100 (again blocking any unabsorbed pump light from the comb) and focused by the lens 102 onto the silicon photodiode detector 104. The detected electrical signal from the detector 104 is fed through a low-pass filter 106 to remove the pulse train from the signal and transmit only the envelope signal. The signal is then viewed on the digitizer 108. In this embodiment, the reference reflection and the target reflection (i.e., the probe and LO pulses arriving at the detector 104) have equal optical energy, and therefore the detected envelopes have equal amplitudes. Therefore, when using a low-power beam, Figure 3 The arrangement can be beneficial (i.e., because it comes from...) Figure 2 The reflection from the glass wedge 42 in the arrangement is weaker than the reflection from the mirror 84.

[0091] refer to Figure 4 and Figure 5 This shows the use of reference Figure 2 An example of raw data collected by the described distance measuring device. Specifically, Figure 4 and Figure 5 The relationship between the filtered electrical signal from the photodiode 58 of such a device and time is shown.

[0092] exist Figure 4 The diagram shows a sequence of multiple envelope pairs. Each peak in the intensity signal is caused by a two-photon absorption effect that occurs when the gating pulse and the probe pulse arrive at the detector together. Figure 4 In the middle, the peaks from the target envelope and the reference envelope almost overlap, but the lower amplitude envelope (i.e., generated by the probe pulse reflecting off the glass wedge 42) and the higher amplitude envelope (i.e., generated by the probe pulse reflecting off the target 44) are still visible.

[0093] Figure 5 It shows Figure 4 The first pair of envelopes is shown in a (time) extended view. The lower intensity peak on the left is caused by the reference pulse returning from glass wedge 42, while the higher intensity peak on the right is caused by the target pulse returning from target 44. Figure 5 The illustration is a further (time-extended) view of the intensity peak caused by the target pulse.

[0094] The time delay of arrival between the reference envelope and the target envelope depends on the distance between the glass wedge 42 (i.e., the reference object) and the target reflector 44. Therefore, time-of-flight distance measurements (i.e., measuring the distance between the reference object and the target) can be performed by identifying the time delays of arrival between the reference and target envelopes. Various techniques can be used to determine the time delays of arrival. For example, the center point of each envelope (i.e., each intensity peak) can be obtained by fitting the envelope to a function, Fourier transform migration analysis can be used to find the time delay, etc. However, given a sufficiently narrow and well-defined envelope, it has been found that simply using the location of the peaks is sufficient. This greatly simplifies the required data processing compared to the complex analysis required when using interferometry-based techniques.

[0095] Next reference Figures 6 to 9 This paper will present a comparison between existing interferometric measurement techniques and the multiphoton effect detection techniques of this invention.

[0096] Figure 6 The diagram illustrates a dual-comb metrology apparatus that allows for comparison of interferometric measurements and detection of multiphoton effects.

[0097] Probe comb 120 and LO comb 122 provide nearly identical probe and gating pulse trains. As described above, the probe comb and LO comb operate at very slightly different repetition rates. The probe pulse train from probe comb 120 passes through quarter-wave plate 124 and half-wave plate 126, which are configured to impart s-polarization to the probe pulses. The s-polarized probe pulses are then reflected from a properly arranged first polarization beamsplitter 128 toward a second polarization beamsplitter 130. The gating pulse train from LO comb 122 passes through quarter-wave plate 132 and half-wave plate 134, which are arranged such that the gating pulses are p-polarized upon reaching the second polarization beamsplitter 130. Thus, the pulse trains from probe comb 120 and LO comb 122 meet at the second polarization beamsplitter 130 with opposite polarizations.

[0098] The combined probe and gating pulse trains are transmitted from the second polarization beamsplitter 130 to the unpolarization beamsplitter 136 via a 1300 nm optical long-pass filter 138, which eliminates any unabsorbed pump light. The combined pulse beams are then split into two beams by the (unpolarization) beamsplitter 136. The transmitted beam is directed to the two-photon detection branch of the device, while the reflected beam is directed to the interference-based detection branch. The transmitted beam is focused onto the silicon photodetector 140 by a lens 142. (Refer to the above reference) Figure 2 and Figure 3The described apparatus, with silicon photodetector 140 configured for two-photon detection, involves a reflected beam passing through polarizer 144 before the probe pulse and gating pulse are focused by lens 148 onto indium gallium arsenide (InGaAs) photodetector 146 to impart matched polarizations to both. InGaAs photodetector 146 has a bandgap capable of detecting incident light (single photon). Specifically, photodetector 146 is able to collect interference patterns formed by the simultaneous arrival of probe and gating pulses with the same polarization.

[0099] Electronic signals from the (two-photon) detector 140 and the (single-photon) photodetector 146 are received by the digitizer 150 after passing through electronic low-pass filters 152 and 154, respectively. Therefore, the digitizer 150 receives the two-photon signal from the silicon photodetector 140 and the single-photon signal from the InGaAs photodetector 146. (Refer to the above text.) Figures 3 to 5 The two-photon signal described provides an intensity envelope sequence formed due to the multiphoton absorption process (i.e., independent of interference effects). In contrast, the signal from the InGaAs photodetector 146 provides a matched sequence of interferograms (i.e., an intensity pattern generated by the optical interference of the gated photon and the probe photon).

[0100] Figure 7a and Figure 7b The above reference is shown. Figure 6 The data collected by the described device. Figure 7a The data collected from the silicon photodetector 140 (i.e., two-photon data) is shown, and Figure 7b The data collected simultaneously from the InGaAs photodetector 146 is shown (i.e., single photon or interferometric measurement data).

[0101] refer to Figure 7a As can be seen, the raw data collected using nonlinear two-photon detection technology (i.e., using photodetector 140) can be easily fitted to a Gaussian function. Therefore, the center of the intensity peak can be easily extracted.

[0102] Figure 7b An interferogram collected using an InGaAs photodetector 146 is shown. According to the prior art system described above, the raw data (plotted as line 160) is an interferogram, in which the time dependence of the electric field can be observed. As explained above, a Hilbert transform is needed to obtain the envelope of the interferogram (curve 162) before fitting a Gaussian function (curve 164) to the Hilbert transform. This Gaussian fitting allows for the estimation of the center point of the interferogram.

[0103] Figure 7a and Figure 7bSeveral advantages of using nonlinear two-photon effect detection according to the present invention are demonstrated. First, it has been found that nonlinear two-photon effect detection can directly collect the intensity envelope without requiring a Hilbert transform. Avoiding the need for a Hilbert transform simplifies the data analysis process and reduces the risk of obtaining inaccurate envelopes. The second advantage arises from the strongly nonlinear response of the two-photon detection process. Figure 7b The full width at half maximum (FWHM) of the Gaussian fit to the interferogram is 0.1 ms, while Figure 7a The full width at half maximum (FWHM) of the Gaussian fit to the envelope is only 0.05 ms. Therefore, it can be seen that the secondary response of the two-photon absorption process leads to an envelope with a more significant peak. This facilitates a more precise determination of the center point of the peak compared to the analysis of equivalent interferograms.

[0104] Next reference Figure 8 This will explain the further advantages of using two-photon detection. As mentioned above, it should be remembered that... (Refer to...) Figure 6 The probe comb 120 and LO comb 122 of the described apparatus are free-running mode-locked lasers. The time dependence of the electric field associated with the optical pulses generated by such mode-locked lasers can be described as a fast sinusoidal oscillation called the carrier multiplied by a more slowly changing envelope function. The relative position between the carrier and the envelope typically changes as the pulse propagates through the medium. In a mode-locked laser, a pulse train is typically generated by a single pulse circulating in a laser resonator. Whenever this pulse hits the output coupler, a decaying copy of that pulse is emitted as the laser output. Typically, there is a specific change in the carrier envelope offset phase during each round trip, which can be hundreds or thousands of radians. Therefore, each emitted pulse can have a different carrier envelope phase. As described above, in existing dual-comb systems relying on interferogram analysis, it is necessary to provide a phase control loop to maintain an invariant offset in the carrier envelope phase of the pulses generated by the two combs. This is achieved by locking the carrier envelope offset frequency (f) of the two combs. CEO The requirements for implementation are complex and have been found to be impractical for industrial applications. Therefore, using a free-running comb (which can be detected using two-photon) can offer significant advantages.

[0105] refer to Figure 8 The three interferograms (i.e., single-photon signals) generated by the InGaAs photodetector 146 are shown in the top plot. The three envelopes (i.e., two-photon signals) generated by the Si photodetector 140 are shown in the bottom plot. Each single-photon signal (interferogram) is collected simultaneously, and the two-photon signal (envelope) is shown directly below it. Correlated envelope / interferogram pairs are collected at three different time points.

[0106] The phase of the interferogram (i.e., in the upper curve) is different at different time points because the free-running combs are used to generate the probe and LO pulse trains. In other words, the carrier envelope offset frequencies (f0 and f1) of the two combs are different. CEO The interferogram varies in an uncontrolled manner. It can be seen that the first interferogram (on the left-hand side) has multiple fringes, the second (middle) interferogram has fewer fringes, and the third (on the right-hand side) interferogram has very few fringes. Therefore, the quality of the Gaussian fit, and consequently the accuracy of any timing measurement, will vary depending on the uncontrolled changes in the phase of the carrier envelope of the two frequency combs. In particular, the example illustration showing only a few fringes illustrates the risk of obtaining an ambiguous interferogram at a specific point in time. Data analysis techniques used to determine the midpoint of the interferogram envelope may use such an ambiguous interferogram to provide distance measurements with lower accuracy compared to interferograms including multiple fringes.

[0107] Compared to the interferogram, it can be seen that the envelope collected via nonlinear two-photon detection is unaffected by the phase change of the carrier envelope of the frequency comb. This is because multiphoton absorption is not an interference effect. Therefore, nonlinear two-photon detection has inherent advantages over traditional dual-comb metrology techniques that rely on extracting the envelope of the interference pattern. For nonlinear two-photon detection, since the comb phase is independent of the detected signal, the requirement for a perfectly stable comb is removed. Therefore, the use of nonlinear two-photon detection is a much simpler and more ingenious solution compared to the optical cross-correlation or balanced cross-correlation techniques previously used by others to attempt to overcome the need for a phase-stable frequency comb, as described above.

[0108] Next reference Figure 9 The diagram shows seven pairs of interferograms (top chart) and two-photon detection envelopes (bottom chart). Each interferogram was collected simultaneously with the two-photon detection envelope shown below it. The interferogram and envelope data collected simultaneously for each pair were analyzed using the difference (Δf) in the detector / LO repetition rate indicated above the correlation interferogram. rep To obtain it.

[0109] As explained above, Δf rep The value determines the rate (i.e., beat frequency) of the LO pulse gating probe pulse. Typically, for existing dual-comb metrology techniques based on photonic interference effects, a lower Δf... rep This value corresponds to a higher effective optical sampling rate, which helps to produce a clear interferogram for high-precision data analysis. And a higher Δf... rep Value and faster update rate (update rate T) update equal to Δf rep Corresponding to the reciprocal of Δf, this allows for rapid scanning of distance measurements. Therefore, Δf repThe selected value represents a trade-off between accuracy and update rate in existing (interference-based) dual-comb metrology. However, in conventional dual-comb metrology, Δf rep The value of is ultimately constrained by the aliasing limit to avoid signal distortion. Specifically, the aliasing limit (Δf) is... lim )equal:

[0110]

[0111] Among them, f rep Δυ is the pulse repetition rate of the probe comb, and Δυ is the frequency bandwidth of the comb. The frequency bandwidth (Δυ) is given by the following formula:

[0112]

[0113] Where c is the speed of light, Δλ is the optical bandwidth of the comb, and λ0 is the center wavelength.

[0114] In this example, the probe laser has a repetition rate of 78.87 MHz, a center wavelength of 1557 nm, and an optical bandwidth (i.e., the absolute full width of the pulse) of approximately 15 nm, resulting in an aliasing limit of approximately 1.7 kHz.

[0115] Figure 9 It is clearly shown that with Δf rep As Δf increases, the shape of the interferogram becomes less defined, and with Δf rep Beyond the approximate aliasing limit, the interferogram becomes distorted. However, the envelope simultaneously collected by nonlinear two-photon detection is similar in shape to Δf. rep The increase in rate is consistent, and a clear boundary is maintained even when the aliasing limit is exceeded. Therefore, using nonlinear two-photon detection, the update rate can exceed the aliasing limit described above without compromising measurement accuracy. It should be noted that for nonlinear two-photon detection, there is indeed a Δf... rep Beyond this limit, the probe pulse and LO pulse cross each other too quickly, producing a less defined envelope. However, for any comb pair used in dual-comb metrology, this value will exceed the limit of existing (interference-based) techniques.

[0116] Next reference Figure 10 The illustration shows an alternative embodiment of the invention for measuring distances through turbulent media and / or to non-cooperative targets.

[0117] An ultrashort probe pulse train from probe comb 180 is optically coupled into fiber circulator 182. The probe pulses exit fiber circulator 182 via first fiber collimator 184. The probe pulses are partially reflected from the (back) reference surface 186 of glass wedge 188 and also partially reflected from target mirror (or rough target) 190. The returned or reflected probe pulses (i.e., pulses reflected from reference surface 186 and target mirror 190) are coupled back into fiber circulator 182 via first fiber collimator 184. These reflected pulses pass through fiber amplifier 192 before exiting the fiber via second fiber collimator 194. The reflected pulse train passes through quarter-wave plate 196 and half-wave plate 198, which are optimized to impart s-polarization to the pulses when they reach polarization beam splitter 200. The LO comb 202 generates an ultrashort gated pulse train that passes through a quarter-wave plate 204 and a half-wave plate 206, which are optimized to impart p-polarization to the pulses when they reach the polarization beamsplitter 200. Therefore, pulses from the probe comb 180 and the LO comb 202 meet at the polarization beamsplitter 200 with orthogonal polarization.

[0118] The combined train of probe and gating pulses exiting the polarization beam splitter 200 passes through an optical high-pass filter 208 (to remove any unabsorbed pump light from the comb) and is focused by lens 212 onto a Si photodetector 210. As described above, the Si photodetector 210 provides a two-photon effect detector. The electrical signal generated by the photodetector 210 passes through an electronic low-pass filter 214 to remove the pulse train components and is observed on a digitizer or oscilloscope 216.

[0119] refer to Figure 10 The described embodiment is advantageous because it enables optical amplification of the reflected (returned) probe signal. This allows for the directing of lower-intensity probe pulses to the target and / or means that the target's reflectivity can be lower than other possible reflectivities. In interferometric systems, the use of optical amplification of reflected probe pulses is typically impossible, where any such amplification alters the wavefront quality and corrupts the carrier envelope phase properties of the reflected probe pulse. However, two-photon detection arrangements are unaffected by any such corruption of the phase properties of ultrashort pulses.

[0120] Next reference Figure 11 , will describe reference Figure 2 A variant of the described device that allows for the determination of distances to multiple targets in parallel. Figure 2 Components of the device that also exist in the apparatus are given the same reference numerals.

[0121] and Figure 2The arrangement is identical, with the probe pulses generated by the probe comb 30 being p-polarized as they pass through the first quarter-wave plate 34 and the first half-wave plate 36. The p-polarized probe pulse train passes through the first polarization beam splitter 38 and the second quarter-wave plate 46 before being partially reflected by the reference surface portion of the glass wedge 42. The pulse train passing through the glass wedge 42 passes through a scattering optics (DOE) 230, which scatters (splits) the target beam into three target beams. Each of the three target beams is reflected from one of the first, second, and third target mirrors 232, 234, and 236. Alternatively, a retroreflector can be used instead of a mirror to maintain the polarization state of the probe pulses. Reflections from the glass wedge 42 (i.e., the reference pulse) and from each of the three target mirrors (i.e., the three target pulse trains) again pass through the quarter-wave plate 46, thus giving all returning pulses s-polarization. The s-polarized reflected pulses meet the p-polarized gating pulses at the second polarization beam splitter 48, and the combined beam is focused by the lens 56 onto the Si photodetector 58. The electronic signal from detector 58 is transmitted to digital converter 62 after being low-pass filtered.

[0122] Three target pulse trains (i.e., target pulse trains reflected from each of the three target reflectors 232, 234, and 236) and a reference pulse train (i.e., reflected from the wedge 42) are gated by a gating pulse train. As explained above, a peak appears in the detected intensity signal when the gating pulse and the probe pulse arrive at the detector together. In this arrangement, groups comprising four peaks or envelopes are repeated sequentially. Each group of four envelopes includes a reference envelope (i.e., generated by the probe pulse reflected from the glass wedge 42) and three target envelopes (i.e., generated by the probe pulse reflected from each of the three targets 232, 234, and 236). The arrival time difference of each target envelope relative to the reference envelope allows the measurement of the distance to each of the three targets. It should be remembered that the use of three target bundles described above is merely an example, and of course, similar arrangements can be implemented with different numbers of target bundles.

[0123] Next reference Figure 12 , will describe reference Figure 2 A variant of the described device. The optical arrangement is consistent with the reference. Figure 2 The optical arrangement is the same as described, but a timer-based arrangement is used instead of a signal digitizer to analyze the electrical signal generated by photodiode 58.

[0124] In particular, Figure 12A microcontroller 250 is shown that receives an electrical signal from a photodetector 58 after the electrical signal passes through an electrical low-pass filter 60. The microcontroller 250 includes first and second timers and is capable of detecting sharp rising and / or falling edges of intensity envelopes occurring in two-photon effect detection in real time. The arrival of a (first) reference envelope is detected by the microcontroller 250, which starts the first timer. The detection of a (first) target envelope stops the first timer and starts a second timer. The arrival of each subsequent envelope stops the running timer and starts another timer. The duration recorded by the first and second timers provides the time-of-flight of the distance between the reference optics and the target optics. The result is a simple, real-time lidar system.

[0125] In this arrangement, the nonlinear response of two-photon detection means that the envelope has a steep rising slope, which can be easily detected (i.e., by starting / stopping the timer). Therefore, variations of the conventional lidar using a dual-comb setup are also provided. The rising slope of the envelope produced by two-photon detection is steeper than that of a single pulse typically detected in lidar, thus providing higher resolution range measurements. In some cases, the use of such a timer-based system may also be preferred over the one described above. Figure 2 The digitizer 62 used in the described device performs data analysis.

[0126] Next reference Figure 13 , Figure 14a and Figure 14b The results of experiments conducted using the distance measuring device according to the present invention will be described.

[0127] First refer to Figure 13 Use the above reference Figure 2 The described device is used to measure the distance between a reference object and a target object, spaced approximately 12 cm apart. Multiple distance measurements were performed. Figure 13 The Allan deviation (accuracy) of the measurement is shown in relation to the averaging time.

[0128] As described above, the update rate is equal to the difference in comb repetition rate (Δf). rep The difference is 4 kHz. For a minimum acquisition time of 250 μs (set by a repeatability difference of 4 kHz), the measurement accuracy is 17.8 μm. Accuracy is scaled by the square root of the sample size, reaching submicron accuracy within a 0.1 s averaging time. After a 1 s averaging time, accuracy reaches 150 nm, sufficient to transfer measurements to interferometric measurements with a 633 nm HeNe laser. After a 2 s averaging time, accuracy reaches 93 nm, which is less than 10 times the accuracy caused by atmospheric fluctuations. -7 Uncertainty limits.

[0129] Referring to Figure 14, the above reference is used again. Figure 2 The described apparatus was used to perform a second experiment to test the linearity and accuracy of the metrology technique. The target mirror was mounted on a translation stage, and the distance to the target mirror was measured in discrete steps within a 1 cm translation range. The measurement results were recorded with an average time of 0.5 s. The Renishaw XL-80 interferometric system also measured the mirror's translation to provide ground-based data.

[0130] Figure 14a The measured distance is shown in relation to the actual data, and a linear relationship is also shown. Figure 14b The residual error between the measured values ​​and the fitted straight line is shown, indicating that the technique has an accuracy of ±4 μm.

[0131] refer to Figure 15 Further alternative configurations of the distance measurement device of the present invention will be described below. As explained below, this embodiment efficiently utilizes available laser energy and has separate paths for detecting reference and target pulses. A probe comb 300 generates ultrashort probe pulse trains (i.e., forms probe beams), which are transmitted to a first polarization beam splitter 302 via a first polarization rotator 304. In this example, the first polarization rotator 304 is a mechanically rotatable half-wave plate, but it can alternatively include a liquid crystal cell or other suitable means for polarization control.

[0132] The p-polarized component of the probe beam passes through the first polarization beam splitter 302 and is guided along a first path via a first quarter-wave plate 308 toward the target mirror 306. The s-polarized component of the probe beam is reflected by the first polarization beam splitter 302 and is guided along a second path via a second quarter-wave plate 312 toward the reference mirror 310. Probe pulses reflected from the target mirror 306 (i.e., the target pulse train) and probe pulses reflected from the reference mirror 310 (i.e., the reference pulse train) return to the first polarization beam splitter 302, where these probe pulses are combined into a single beam (i.e., the return pulse train) guided toward the second polarization beam splitter 314.

[0133] The LO comb 320 generates an ultrashort gating pulse train (i.e., forms an LO beam), which passes through a second polarization rotator 322 in the form of a mechanically rotatable half-wave plate (note that this may alternatively include a liquid crystal cell or other suitable device for polarization control) toward a second polarization beam splitter 314.

[0134] The p-polarized component of the LO beam passes through the second polarization beamsplitter 314 toward the first silicon photodiode detector 324. The s-polarized component of the probe beam (i.e., the target pulse train) is reflected by the second polarization beamsplitter 314 and is therefore also directed toward the first silicon photodiode detector 324. For the reasons outlined above, a high-pass optical filter 326 and a lens 328 are provided between the second polarization beamsplitter 314 and the first silicon photodiode detector 324. It can thus be seen that orthogonally polarized target pulses and gating pulses reach the first silicon photodiode detector 324.

[0135] The s-polarization component of the LO beam is reflected from the second polarization beamsplitter 314 and guided toward the second silicon photodiode detector 330. The p-polarization component of the probe beam (i.e., the reference pulse train) is transmitted through the second polarization beamsplitter 314 and also guided toward the second silicon photodiode detector 330. For the reasons outlined above, a high-pass optical filter 332 and a lens 334 can be disposed between the second polarization beamsplitter 314 and the second silicon photodiode detector 330. For convenience, a beam manipulation (direction) mirror 336 is also provided. Thus, it can be seen that orthogonally polarized reference pulses and gating pulses reach the second silicon photodiode detector 330.

[0136] The electrical signals detected from the first silicon photodiode detector 324 and the second silicon photodiode detector 330 are fed to the microcontroller 340 via a first low-pass filter 342 and a second low-pass filter 344, respectively. Thus, the two-photon effect reference signal and the target signal are detected via two separate channels, but the microcontroller 340 can perform an analysis similar to that described above, where only a single two-photon effect detector is used. For example, the microcontroller 340 can use peak detection to start a timer to implement a lidar-type arrangement. Alternatively, an analyzer can be provided to analyze the collected signals to determine the relative generation times of the signal (e.g., reference and target) peaks.

[0137] Importantly, the provision of a first polarization rotator 304 and a second polarization rotator 322 enables control over the relative intensities of various beams within the apparatus. Specifically, adjusting the first polarization rotator 304 controls the relative proportions of the probe beams guided to the target mirror 306 and the reference mirror 310. In other words, the system can balance the laser power downwards along the reference and target arms by rotating the polarization of the probe beams using the first polarization rotator 304. This allows control over the relative intensities of the target and reference pulses and allows for the interpretation of variations in the amount of light returning from the target. For example, influences such as the reflectivity of the target may alter how much light is needed to generate the best possible measurement signal. The second polarization rotator 322 provides similar control over the local oscillator beam, thereby allowing appropriate control (e.g., balancing) of the relative amounts of the LO beam received by the first photodiode 324 and the second photodiode 330.

[0138] refer to Figure 16 Further alternative configurations of the distance measuring device of the present invention are described.

[0139] The probe comb 400 generates an ultrashort probe pulse train (i.e., forms a probe beam), which is transmitted to the first polarization beam splitter 402 via the first half-wave plate 404. The p-polarization component of the probe beam passes through the first polarization beam splitter 402 and is guided along a first path via the first quarter-wave plate 408 toward the target mirror 406. The s-polarization component of the probe beam is reflected by the first polarization beam splitter 402 and is guided along a second path via the second quarter-wave plate 412 toward the reference mirror 410. The probe pulses reflected from the target mirror 406 (i.e., the target pulse train) and the probe pulses reflected from the reference mirror 410 (i.e., the reference pulse train) return to the first polarization beam splitter 402, where these probe pulses are combined into a single beam (i.e., the return pulse train) guided toward the second polarization beam splitter 414.

[0140] The LO comb 420 generates an ultrashort gating pulse train (i.e., forms an LO beam), which passes through the second half-wave plate 422 toward the second polarization beam splitter 414. The p-polarized component of the LO beam passes through the second polarization beam splitter 414 toward the first silicon photodiode detector 424 (i.e., the first multiphoton effect detector element). The s-polarized component of the probe beam (i.e., the target pulse train) is reflected by the second polarization beam splitter 414 and is therefore also guided toward the first silicon photodiode detector 424. For the reasons outlined above, a high-pass optical filter 426 and a lens 428 are provided between the second polarization beam splitter 414 and the first silicon photodiode detector 424. Thus, it can be seen that orthogonally polarized target pulses and gating pulses reach the first silicon photodiode detector 424. The high-pass optical filter 426, the lens 428, the first silicon photodiode detector 424, and the first low-pass filter 442 together provide a target detection channel 452.

[0141] The s-polarization component of the LO beam is reflected from the second polarization beamsplitter 414 and guided toward the second silicon photodiode detector 430 (i.e., the second multiphoton effect detector element). The p-polarization component of the probe beam (i.e., the reference pulse train) is transmitted through the second polarization beamsplitter 414 and also guided toward the second silicon photodiode detector 430. For the reasons outlined above, a high-pass optical filter 432 and a lens 434 can be disposed between the second polarization beamsplitter 414 and the second silicon photodiode detector 430. For convenience, a beam manipulation (direction) mirror 436 is also provided. Thus, it can be seen that orthogonally polarized reference pulses and gating pulses arrive at the second silicon photodiode detector 430. The high-pass optical filter 432, lens 434, second silicon photodiode detector 430, and second low-pass filter 444 together provide a reference detection channel 450.

[0142] Detected electrical signals from reference detection channel 450 and target detection channel 452 are fed to lidar timer circuit 440. Timer circuit 440 uses peak detection to start timer clock 454 to implement a lidar-type arrangement. Specifically, a signal from the reference channel starts timer clock 454, and a signal from the target channel stops lidar timer clock. The time difference measured by timer clock 454 is proportional to the absolute distance.

[0143] It should be remembered that the distance (d) to the target is calculated using the following formula:

[0144]

[0145] This formula can be rewritten as:

[0146]

[0147] In fact, the system uses factors Slowing down the speed of light, c, means that a lower resolution is needed to obtain a high-resolution measurement of the absolute distance, d. For example, the effective speed of light can be slowed down by about 20,000 times and can be considered a "time stretching" factor.

[0148] If the maximum measurement rate is Δf rep The scope is then clearly defined by Given. Therefore, by appropriately choosing f rep probe and f rep LO To tune distance resolution, measurement rate, and defined range.

[0149] The repetition rate f of each laser rep The length of the laser cavity determines the operating range (L). unamb) and the length of the local oscillator (L) LO Related to this, the expression is as follows:

[0150]

[0151] In order to perform the operation within a defined range such as 3m, n osc A fiber laser oscillator cavity with a diameter of approximately 1.5 must be about 2 meters long.

[0152] Now for reference Figures 17 to 19 This will explain how the time between the first reference pulse, the subsequent target pulse, and the next reference pulse can be recorded advantageously. This can be achieved using two LiDAR clocks or timers. In particular, using this information, not only can the distance to the target be calculated, but Δf can also be calculated based on the time between adjacent reference pulses. rep .

[0153] refer to Figure 17 This demonstrates the optical similarity to Figure 16 The embodiments shown have very similar arrangements. Therefore, the same reference numerals are used to identify the reference numerals. Figure 16 The same components present in the described embodiments. Figure 17 The main difference in this embodiment is that the signals output from the reference detection channel 450 and the target detection channel 452 are both fed to the first lidar timer 500 and the second lidar timer 502. These timers 500 and 502 operate using the same time reference obtained from the master clock 504. The timing of the LO comb 420 is also obtained from the master clock 504.

[0154] The first LiDAR timer 500 is started by the reference pulse and stopped by the target pulse, and reports the reference target time (t). RT The second lidar timer 502 starts on the target pulse and stops on the next reference pulse, and reports the target reference time (t). TR ).

[0155] refer to Figure 18 The illustration shows the use Figure 17 Timing information obtained from two timers. The upper trace shows the reference arm signal, and the lower trace shows the target arm signal. The reference target time (t) is shown. RT ) and target reference time (t) TR The value of ) is used for N pulses in two columns. Note that the time difference Δt mentioned in the example above corresponds to t RT .

[0156] Reference target time (t) RT ) and target reference time (t) TRThe sum of t gives the time between reference pulses or the reference-reference time (t). RR The reference mirror 410 is immovable, and therefore the reference pulses will appear at fixed time intervals. The period of these intervals is determined by the time it takes for the two laser combs 400 and 420 to align given their different repetition rates. Therefore, the reference-reference time (t) is... RR The difference in repetition rate is related to the following expression:

[0157]

[0158] Therefore, the distance can be calculated using the following formula:

[0159]

[0160] Because t was measured RT and t TR Both (i.e., via the first lidar timer 500 and the second lidar timer 502) allow for control of Δf without requiring strict control. rep The distance d is determined under the following conditions.

[0161] Next reference Figure 19 The layout and reference are shown. Figure 16 and Figure 17 The similar embodiments described herein. Therefore, the same reference numerals are used to identify the same components present in this embodiment, which are also present in the reference numerals. Figure 16 and Figure 17 The described embodiments.

[0162] Figure 19 The device and Figure 17 The difference with this device is that it directly uses a reference-reference time (t). RR ) signal to compensate Δf rep Value drift. Reference - Reference time (t) RR The signal is obtained by the phase-locked loop (PLL) controller 600 from the signal output by the reference detection channel 450. Specifically, t represents... RR The timing signal is directly fed to a high-pass filter to select higher harmonics, and from there to a phase-locked loop, where the frequency of the phase-locked loop is increased by a factor of many (k in this example). A first lidar timer 500 and a second lidar timer 502 use the resulting timing signal obtained from the PLL controller 600 as a clock reference. In this way, Δf repAny change in the repetition rate will alter the ticking frequency of the reference clock, and thus automatically compensate for any such change in the repetition rate on the distance measurement. This means that the repetition rate of the probe comb 420 does not need to be strictly stabilized, thereby allowing Δf rep As time drifts slightly.

[0163] Next reference Figure 20 It shows the reference Figure 19 The described embodiment is similar to another embodiment, but the timing signal is obtained in a different manner. Therefore, the same reference numerals are used to identify components present in this embodiment, which are also present in the reference numerals. Figure 16 , Figure 17 and Figure 19 The described embodiments.

[0164] Figure 20 The device includes a first glass wedge 700 that guides a light sample generated by a probe comb 400 to a first fast photodiode 702. A second glass wedge 704 is also provided that guides a light sample generated by a probe comb 420 to a second fast photodiode 706. A timing controller 708 receives electrical signals from the first photodiode 702 and the second photodiode 706. The timing controller 708 bandpass-filters each signal to obtain the nth harmonic of each signal, which is in the radio frequency (RF) band. The RF signals are then mixed to generate their beat frequencies, which indicate that any Δf will be followed. rep n drifting Δf rep The n Δf rep The signal can be squared and used as a reference clock for LiDAR timers 500 and 502, thereby compensating for any Δf in the distance measurement. rep drift.

[0165] refer to Figure 20 The described arrangement removes the sensitivity to changes in the position of mirror 410 during measurement, which may occur in the reference. Figure 19 The described layout. Of course, such timing information can be extracted in various alternative ways. For example, the above reference can be analyzed. Figure 3 The digitizer 108 in the described arrangement receives signals to obtain such timing information. Those skilled in the art will also recognize that other techniques may be possible.

[0166] It should be remembered that the embodiments described above are merely examples of apparatuses and methods that can be implemented according to the present invention. For example, although two-photon detection has been described, multiphoton effect detection requiring three or more photons to arrive at the detector simultaneously can also be implemented. Furthermore, it will be possible to prevent optical interference between the gating pulse and the probe pulse from alternative arrangements of cross-polarization described above. For example, the probe comb and the LO comb can output pulse trains in different (e.g., non-overlapping) wavelength ranges, or the incident angles of the probe pulse and the gating pulse on the detector can be arranged to minimize interference effects.

Claims

1. An optical distance measuring device, comprising: At least one optical pulse generator is provided for generating a gating pulse train and a probe pulse train, the gating pulse train having a different repetition rate than the probe pulse train. An optical detection arrangement is provided for directing the detection pulse train to one or more objects and for collecting return detection pulses from the one or more objects. The device is characterized in that it includes a multiphoton effect detector and is configured to direct both the gating pulse train and the return probe pulse to the multiphoton effect detector.

2. The apparatus according to claim 1, wherein, The multiphoton effect detector has a band gap greater than the photon energy of a single photon of the gating pulse and the probe pulse.

3. The apparatus according to any of the preceding claims, wherein, The multiphoton effect detector includes a two-photon effect detector, wherein the combined energy of the gated pulse photon and the probed pulse photon is greater than the band gap of the two-photon effect detector.

4. The apparatus according to claim 1, wherein, The multiphoton effect detector comprises silicon.

5. The apparatus according to claim 1, wherein, The multiphoton effect detector includes multiple detector elements.

6. The apparatus of claim 1, further comprising an analyzer for analyzing the relationship between the output of the multiphoton effect detector and time.

7. The apparatus of claim 1, further comprising a time-of-flight monitor, the time-of-flight monitor including at least one peak detector and at least one timer, the at least one peak detector being configured to detect peaks in the output of the multiphoton effect detector, and the at least one timer being configured to measure the time between the detected peaks.

8. The apparatus according to claim 1, wherein, The at least one optical pulse generator includes a probe mode-locked laser for generating the probe pulse train and a local oscillator mode-locked laser for generating the gating pulse train.

9. The apparatus according to claim 8, wherein, Both the probe mode-locked laser and the local oscillator mode-locked laser are free-running mode-locked lasers.

10. The apparatus according to claim 1, wherein, The optical configuration of the device substantially prevents optical interference between the gating pulse and the probe pulse at the multiphoton effect detector.

11. The apparatus according to claim 10, wherein, The gating pulse has a different polarization state than the returning probe pulse when it reaches the multiphoton effect detector.

12. The apparatus according to any one of claims 10 to 11, wherein, The gating pulse has an optical wavelength different from that of the probe pulse.

13. The apparatus according to claim 1, wherein, The one or more objects include a reference object and one or more remotely located target objects, and the returned probe pulses include a reference pulse returned from the reference object and a target pulse returned from the one or more remotely located target objects.

14. The apparatus according to claim 1, wherein, The optical detection arrangement includes a beam splitter that divides an input detection beam, including the detection pulse train, into multiple output detection beams that are directed to multiple targets. The optical detection arrangement also collects return detection pulses from each of the multiple targets.

15. A method for measuring optical distance, comprising the following steps: (i) Generate a probe pulse train (ii) Generating a gating pulse train, the gating pulse train being generated at a different repetition rate than the probe pulse train. (iii) Direct the probe pulse train toward one or more objects, and collect the returned probe pulses from the one or more objects. (iv) The gating pulse train and the return probe pulse are directed to a detector, wherein the detector is configured to perform multiphoton effect detection.

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