Patent Description:
Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample's spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers.

Dual-comb technique has enabled exciting applications in high resolution spectroscopy, precision distance measurements, and 3D imaging. Major advantages over traditional methods can be achieved with the dual-comb technique. For example, dual-comb spectroscopy provides orders of magnitude improvement in acquisition speed over standard Fourier-transform spectroscopy while still preserving the high-resolution capability. Wider adoption of the technique has, however, been hindered by the need for complex and expensive ultrafast laser systems.

Fourier-transform spectroscopy is a tool for analyzing chemical samples in scientific research as well as the chemical and pharmaceutical industries. Recently, its measurement speed, sensitivity, and precision have been shown to be significantly enhanced by using dual-frequency combs. Moreover, recent demonstrations of inducing nonlinear effects with ultrashort pulses have enriched the utility of dual-comb spectroscopy. However, wide acceptance of this technique is hindered by its requirement for two frequency combs and active stabilization of the combs.

The ability to determine absolute distance to an object is one of the most basic measurements of remote sensing. High precision ranging has important applications in both large-scale manufacturing and in future tight formation flying satellite missions, where rapid and precise measurements of absolute distance are critical for maintaining the relative pointing and position of the individual satellites. Using two coherent broadband fiber-laser frequency comb sources, a coherent laser ranging system that combines the advantages of time-of-flight and interferometric approaches to provide absolute distance measurements, simultaneously from multiple reflectors and at low power, is known. The pulse time-of flight yields a precision of <NUM> with an ambiguity range of <NUM> in <NUM>. Through the optical carrier phase, the precision is improved to better than <NUM> at <NUM>, and through the radio-frequency phase the ambiguity range is extended to <NUM>, potentially providing <NUM> parts in <NUM><NUM> ranging at long distances. However, generally only either the object distance or the object speed can be determined at one time.

Dual-comb measurement techniques have shown great promises in applications that demand accuracy and stability, such as precision spectroscopy and coherent lidar. However, widespread use of dual-comb measurement techniques is currently limited by the requirement of two mode-locked femtosecond laser frequency combs and high-speed, phase-lock loop electronics to create the necessary mutual coherence. Thus, there is a need for better laser frequency combs. There is also a need for measurement techniques that can unambiguously determine the object distance and the object speed in just one measurement.

FMCW LiDAR is yet another promising laser ranging technique. In the FMCW LiDAR system, the object distance is linearly encoded as the measured electrical frequency. Traditionally, the object speed also leads to an offset in the measured electrical frequency and consequently results in ambiguity in distance unless another independent measurement on the object speed is conducted. Using the dual-sideband method, described in various embodiments herein, this problem is solved, and both the object distance and the object speed can be unambiguously determined in just one measurement.

Embodiments of the invention include a dual-comb generation system comprising: a bidirectional mode-locked femtosecond laser configured to generate two laser outputs; a rotation stage having a rotational speed of <NUM>,<NUM> rpm or more; wherein the bi-directional mode-locked femtosecond laser is placed on the rotation stage and the Sagnac effect creates a repetition rate difference between the two laser outputs (<NUM>, <NUM>) in a clockwise direction and a counterclockwise direction, and the rotation stage is coupled to a pump diode; and a fiber coupler configured to combine the two laser outputs. An exemplary dual comb measuring system may include a bi-directional mode-locked femtosecond laser, a high-speed rotation stage, and a fiber coupler. The high-speed rotation stage may be coupled to a pump diode.

This summary and the following detailed description are merely exemplary, illustrative, and explanatory, and are not intended to limit, but to provide further explanation of the invention as claimed. Other systems, methods, features, and advantages of the example embodiments will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description.

The figures provided are diagrammatic and not drawn to scale. Variations from the embodiments pictured are contemplated. Accordingly, illustrations in the figures are not intended to limit the scope of the invention.

The following disclosure describes various embodiments of the present invention and method of use in at least one of its preferred, best mode embodiment, which is further defined in detail in the following description.

In the following description and in the figures, like elements are identified with like reference numerals. The use of"e.g.," "etc.," and "or" indicates non-exclusive alternatives without limitation, unless otherwise noted. The use of "including" or "includes" means "including, but not limited to," or "includes, but not limited to," unless otherwise noted.

Turning to the figures, <FIG> illustrates a bi-directional laser which is inherently compatible with dual-comb measurement techniques 100A alone without an additional laser. As illustrated, the bi-directional laser utilizes the Sagnac effect to create a repetition rate difference between the bi-directional laser outputs <NUM>, <NUM>. Other principles to create a repetition rate difference may also be used. For a single frequency laser gyro in rotation, due to the Sagnac effect, the lasing frequencies in the clockwise direction and the counterclockwise direction will differ by Δfopt = α · fopt · Ω, where α is a constant depending on the laser cavity design, fopt is the lasing frequency when the gyro is at rest, and Ω is the angular rotation speed.

Similarly, as illustrated in <FIG>, if a bi-directional mode-locked femtosecond laser is put on a high-speed rotation stage <NUM> coupled to a pump diode <NUM>, the Sagnac effect may also introduce a repetition rate difference between the laser outputs <NUM>, <NUM> in the clockwise direction and the counterclockwise direction. The repetition rate difference may be proportional to the angular speed of the rotation stage <NUM> and it can be expressed as Δfrep = α · frep · Ω, where α is the same cavity design dependent constant and frep is the repetition rate when the system is at rest. As illustrated in <FIG>, there is no need to build two mode-locked femtosecond lasers. The two laser outputs <NUM>, <NUM> are combined as shown in <FIG> using a standard fiber coupler, and then we have the source for dual-comb measurements. Any other method of combining the two laser outputs <NUM>, <NUM> may also be used. In the standard two-laser implementation of dual-comb measurement, the two laser cavities fluctuate independently with their noise completely uncorrelated. Thus, high speed feedback electronics may be necessary to lock the two otherwise independent cavities and ensure the mutual coherence between the two lasers. In some embodiments, the bi-directional laser outputs <NUM>, <NUM> share the same cavity, and thus any linear cavity fluctuation is equally experienced by the two bi-directional laser outputs. Because of this common noise characteristics, the two laser outputs <NUM>, <NUM> may be mutually coherent in nature even without the need of high-speed phase-lock loop electronics. The rotation stage <NUM> may also include a fiber rotary joint <NUM> to decouple the pump fiber from the cavity rotation.

Recently there is an increasing interest in applying bidirectional fiber lasers to dual-comb measurement systems due to the reduced system complexity and lower cost. Widespread use of dual-comb measurement techniques is currently limited by the requirement of two mode-locked femtosecond laser frequency combs and high-speed phase-lock loop electronics. In some embodiments, by replacing the two mode-locked lasers with a bidirectional fiber laser, the cost of laser may be reduced by half. In addition, high speed phase-lock loop electronics may no longer be necessary as discussed in the previous paragraph, cutting down the system complexity and cost even more. Currently, the existing technology has the fiber laser cavity at rest, thus requiring the cavity to be asymmetric. Further, unequal nonlinearity has to be introduced for the repetition rates of the two directions to be different. Due to the asymmetric cavity and the required nonlinearity, cavity noise experienced by the two directions can no longer be perfectly canceled (only linear cavity fluctuation is equally experienced by the two directions). Thus, the bidirectional laser outputs still gradually drift away from each other and mutual coherence will be lost unless a slow feedback is implemented. Furthermore, to ensure the laser stability, the asymmetry and the nonlinearity cannot be set too high and thus the repetition rate difference is typically limited to <<NUM>, which consequently limits the data acquisition rate of the dual-comb measurement system.

On the other hand, in some embodiments, the bi-directional laser using the dual-comb measurement system may utilize the Sagnac effect, which is linearly controllable by the speed of the rotation stage. As the repetition rate difference may not depend on the cavity asymmetry and the nonlinearity anymore, in some embodiments, the gradual loss of mutual coherence as well as increase the data acquisition rate can be eliminated. In some embodiments, a readily available motorized rotation stage with a speed of <NUM>,<NUM> rpm (Ω) may be used so that a repetition rate difference (Δfrep) of <NUM>, i.e., more than an order of magnitude enhancement compared to the prior art may be achieved. Any other kind of rotation stage may also be used. With a high-speed rotation stage that has a speed of <NUM>,<NUM> rpm, the repetition rate difference may be further increased to the level of <NUM>. Further, as the repetition rate is linearly proportional to the rotation speed (Δfrep = α · frep · Ω), it may be tuned easily by only changing the speed of the rotation stage and recalibrated by keeping track of the motor's rotation speed. Besides, the high-speed rotation increases the system's moment of inertia and thus makes the whole system more stable against any disturbance in the environment (just like a spinning bullet has a more stable projectile).

<FIG> illustrate graphs associated with the standard carrier suppressed with a single side band that depict traditional FMCW LiDAR principle. <FIG> illustrates a graph 200A depicting that the carrier and the negative sideband are suppressed and only the positive sideband is utilized to do the ranging measurement. <FIG> illustrates the optical frequency graph 200B of a positive sideband as a function of time, showing a positive slope of the frequency sweep (Δ/T). <FIG> illustrates a standard FMCW LiDAR result graph 200C with a weakly reflected object at <NUM> (L=<NUM>) and a strongly reflected object at <NUM> (L=<NUM>). In the FMCW LiDAR system, the object distance is linearly encoded as the measured electrical frequency with the equation fM = <MAT> where c is the speed of light and fD is the Doppler frequency resulting from the object's speed. As shown by the equation, the object speed leads to an offset in the measured electrical frequency and consequently results in distance ambiguity unless another independent measurement on the object speed is conducted.

<FIG> illustrate graphs associated with a carrier suppressed dual sideband that depict exemplary dual-sideband FMCW LiDAR principle. <FIG> illustrates a graph 300A of when only the carrier is suppressed in the dual-sideband FMCW LiDAR. Both the positive and the negative sidebands may be utilized to do the ranging measurement. <FIG> illustrates an optical frequency graph 300B of both sidebands as a function of time. There may be a simultaneous positive slope of the frequency sweep and a negative slope of the frequency sweep. <FIG> illustrates an exemplary result graph 300C from an exemplary dual-sideband FMCW LiDAR, again with a weakly reflected object at <NUM> (L=<NUM>) and a strongly reflected object at <NUM> (L=<NUM>). For each object, there may be two measured electrical frequencies, one upshifted fM,u and one downshifted fM,d by the Doppler frequency. In some embodiments, the dual-sideband method may be used to simultaneously and unambiguously determine both the object distance and the object speed in just one measurement. The object distance and speed may be calculated by averaging and differencing the two electrical frequencies (fM,u and fM,d), respectively.

<FIG> illustrates a schematic view of a dual-sideband FMCW LiDAR system 400A. A single frequency diode laser <NUM> may be fed to an electro-optic amplitude modulator ("EOM") <NUM>. The EOM <NUM> may be used to create the two sidebands from the single frequency diode laser <NUM>. The EOM <NUM> bias voltage is carefully chosen to suppress the carrier frequency. The Erbium doped optical amplifier <NUM> may then be used to boost the optical power to 3W. The 2D scanning unit <NUM> may be controlled by a computing system <NUM> to direct the light to the region of interest and eventually form LiDAR images. Balanced detection technique <NUM> may also be incorporated to improve the measurement sensitivity so the dual-sideband FMCW LiDAR system 400A can measure an object with <NUM>% reflectivity at a distance of <NUM>.

An exemplary LiDAR system that is currently available is the one from Velodyne. The Velodyne LiDAR system involves mechanical rotation. It uses sixty-four lasers and sixty-four detectors to cover different vertical angles. Sixteen lasers and thirty-two detectors are in one group. However, the major drawback of this LiDAR system is the lower rotation speed and complicated design of the LiDAR system. Another available LiDAR system is the one from Quanenergy. It uses an optical phase array to scan for objects, which steers the light direction by controlling the phase of each antenna. However, the major drawback of that system is that as the spot quality is poor, it is difficult to detect an object using the system over long distances. <FIG> illustrates a dual sideband FMCW LiDAR system <NUM>, according to exemplary embodiments of the present invention. The LiDAR system mainly includes three parts: modulated light generation unit, transceiver unit <NUM>, and control and processing unit <NUM>.

The transceiver unit <NUM> have one or more transceiver terminals <NUM> and <NUM>. It may have laser and control signal <NUM> and data links <NUM>. It may also have a control and signal processing unit <NUM>. The control and signal processing unit <NUM> may be separated from the transceiver unit <NUM>, which makes the in-car system layout of the LiDAR system <NUM> more flexible. The transceiver unit <NUM> may be placed on top of the vehicle while the control and signal processing unit <NUM> may be placed in the car. And the control signal may be delivered to the transceiver unit through long electrical cable, while the receiving light signal is sent back to the signal processing unit <NUM> through a long SMF-<NUM> fiber.

<FIG> illustrates a transceiver terminal <NUM> used in a LiDAR system, according to embodiments of the present invention. The transceiver terminal may use a two-axis control mirror system with an x-axis control mirror <NUM> and a y-axis control mirror <NUM> to achieve high-speed 3D scanning and fast adjustment of the scanning angles. The transceiver terminal <NUM> may also have a detection module <NUM>. The laser and control signal <NUM> may be deflected using the two-axis control mirror system to the object <NUM>. The data links <NUM> may pass through the detection module to the object <NUM>.

<FIG> illustrates a control data processing center <NUM> used in a LiDAR system, according to embodiments of the present invention. The center <NUM> may be used to encode high dimension information on a traditional laser signal, which can get more information of an object such as velocity. With this high-dimensional information from the LiDAR system, less speculation and conjecture are necessary for an artificial intelligence unit to make sense of its situation in a complex environment. In other words, one can reduce the computation load of the artificial intelligence unit because of the enhanced sensing capability of the invented LiDAR system.

<FIG> illustrates a schematic diagram <NUM> of a dual-sideband FMCW LiDAR system with a modulated light generation unit, transceiver unit, and control and processing unit. In the modulated light generation unit, a continuous wavelength (CW) diode laser <NUM> centered at <NUM> is fed to a <NUM> electro-optic amplitude modulator (EOM) <NUM>. By sending a radio frequency (RF) signal into the EOM <NUM>, the EOM <NUM> generates two equal-intensity sidebands from the CW diode laser <NUM>. A voltage-controlled oscillator (VCO) <NUM> is applied here to generate RF signal for frequency modulation of the CW laser. The driving signal of the VCO <NUM> is a <NUM> pseudo-sawtooth signal with a voltage output ranging from <NUM> V to <NUM> V, yielding a RF signal sweeping from <NUM> to <NUM>. The <NUM> is chosen such that the maximum detection range could theoretically reach <NUM>. The system could also change the sweeping frequency form <NUM> to <NUM> for different detection range while maintaining the same resolution. The wavelength and bandwidth may be selected as long as they complement the features descried herein. By carefully choosing the bias voltage through a power supply with an mV-level accuracy, the carrier frequency is maximally suppressed, so the two sidebands attain maximum intensity. The modulated optical signal may then be amplified by an Erbium-doped fiber pre-amplifier <NUM> to <NUM> mW and is split into two paths by a <NUM>:<NUM> fiber coupler <NUM>. One path is fed to a high-power Erbium-doped fiber amplifier (EDFA) <NUM> and then be boosted to <NUM> W, the other path is sent into a <NUM>×<NUM><NUM>:<NUM> fiber coupler <NUM>, used as a local reference for ranging measurement. The output of EDFA <NUM> is connected to the transceiver unit from the bottom to the top of a vehicle via a long SMF-<NUM> fiber and convert to free space beam through a collimator. Then the light beam is expanded to a <NUM>-cm-diameter beam by a beam expander for maximum signal collection. The larger the beam, the better. A polarization beam splitter (PBS) <NUM> is implemented for collinear detection. Then the output signal is optimized to transverse electric (TE) polarization for maximum utilization of the intensity via a half wave plate (HWP). Then the light is directed to the 2D scanning unit <NUM> controlled by the control and processing unit. The 2D scanning unit <NUM>, as illustrated in <FIG> consists of two components, one is a galvo mirror <NUM> for vertical steering and a rotating octagon mirror <NUM> for horizontal steering of the light. The galvo rotational angle is set to achieve up to <NUM>° vertical angle of view. It may also have a zoom-in function for far object detection that could be achieved by changing the rotational angle range of the galvo mirror <NUM>. The position of the galvo mirror <NUM> and the octagon mirror <NUM> is carefully set up to achieve a <NUM>° horizonal angle of view. The scan rate of the galvo mirror <NUM> is determined by a global trigger rate ranging from <NUM> to <NUM>. The rotational speed of octagon mirror <NUM> is set to <NUM> rpm, yielding four hundred horizontal scanning lines per second in total. And by changing the trigger rate from <NUM> to <NUM>, the resolution of the rendering image could be switched from <NUM> lines/frame to <NUM> lines/frame. When the resolution is <NUM> lines/frame, there may be <NUM> points per line and the vertical spatial resolution could reach <NUM>° and horizontal spatial resolution could reach <NUM>°, thus improving the clarity of the image.

As illustrated in <FIG>, the control and processing unit may include a balanced photodetector (BPD) <NUM>, a high-speed DAQ card, a high-speed processor, a two-channel arbitrary waveform generator and a two-channel signal generator. The balanced detection technique is incorporated to improve the measurement sensitivity so the dual-sideband FMCW LiDAR system could measure an object with <NUM>% reflectivity at <NUM>. The balanced detector may receive two signal paths and cancel out the common noise. The BPD may only detect the signal difference between two paths. The LiDAR system may receive about -<NUM> dBm power from an object with <NUM>% reflectivity at <NUM>, which reaches the noise equivalent power of the balanced photodetector. The received signal is coupled with the local reference signal via a <NUM>×<NUM><NUM>:<NUM> fiber coupler <NUM> and detected by the BPD <NUM> with <NUM> bandwidth, which matches with the frequency modulation range of the laser. Any other matching pair of bandwidth and modulation frequency may be used. The polarization of the local reference path is optimized through three rotating paddles of a fiber polarization controller and the power of the two inputs of BPD is adjusted to perfectly equalize by inserting a fiber variable optical attenuator for optimal signal to noise ratio. Then the signal is acquired by the DAQ card with a <NUM> sampling rate, and then a graphic card assisted real-time fast Fourier transform (FFT) is applied to detect the two electrical frequency peaks of both sidebands, <MAT> and <MAT>, where Δ = VCO sweeping range × VCO sweeping rate. Then the object distance and speed can be simultaneously and unambiguously calculated by averaging and differencing the two frequencies, respectively. At last, the processed signal may be used for point of cloud generation of the region of interest, and real-time rendering on the interface in the vehicle. The arrayed-waveguide grating (AWG) offers control signals for the VCO <NUM> and galvo mirror. The signal for the VCO <NUM> may be customized sawtooth signal with higher order terms to compensate the nonlinear sweeping of VCO, and the signal for galvo mirror is a tilted triangular waveform. The shape of the signal could be anything, as long as it is calibrated in the postprocessing. The two-channel signal generator offers a <NUM>-<NUM> pulse signal with <NUM>% pulse duration to the DAQ card to acquire data, and a <NUM> TTL signal with <NUM>% duty ratio with maximum <NUM> V output to control the octagon mirror. Accordingly, in some embodiments, the light generation unit and control and signal processing unit described herein and implemented in the car (or any other transportation device, such as planes, ships, etc.) may be under the seat or somewhere else, and the transceiver unit may be mounted on top of the vehicle. The light may be delivered to the transceiver unit through a long fiber cable and the received signal may be be sent back to the control and signal processing unit through the long fiber. The 2D scanning unit is controlled by the control unit through a long BNC cable. Under 2D scanning, the received signal may be rendered to a real-time 3D point of cloud and show up on the interface, which may be a display device net to the driver. The transportation device may have the currently available features to transmit the information, both audio and video, to the driver.

Claim 1:
A dual-comb generation system comprising:
a bidirectional mode-locked femtosecond laser configured to generate two laser outputs (<NUM> and <NUM>);
a rotation stage (<NUM>) having a rotational speed of <NUM>,<NUM> rpm or more;
wherein the bi-directional mode-locked femtosecond laser is placed on the rotation stage and the Sagnac effect creates a repetition rate difference between the two laser outputs (<NUM>, <NUM>) in a clockwise direction and a counterclockwise direction, and the rotation stage is coupled to a pump diode (<NUM>); and
a fiber coupler configured to combine the two laser outputs.