TECHNICAL FIELD

Examples described herein relate to the field of optical distance measurement, including light detection and ranging (LiDAR) systems and methods, such as frequency-modulated continuous-wave (FMCW) LiDAR systems and methods, or and length metrology. Examples of systems and methods which may provide advantageous digitization are described.

BACKGROUND

FMCW ladar generally refers to a form of coherent laser distance measurement that may use substantially linear frequency modulation of a laser output, which is referred to as a form of frequency ‘chirp’, to achieve distance measurements. The laser frequency may be directly chirped, or the frequency chirp may be imparted to the laser frequency by a modulator that may be external to the laser. FMCW ladar uses optical heterodyne detection, which can provide quantum-noise-limited measurement signals, and allow measurements with a large dynamic range of signal powers due the square root dependence of the FMCW signal on the scattered optical power received from the target. If desired, direct modulation of the laser output frequency can result in large chirp bandwidths, and therefore, high-resolution distance measurements due to the well-known relationship, ΔR=c/2B, where ΔR is the range resolution, c is the speed of light and B is the chirp bandwidth. Chirped sideband modulation using an RF chirp and, for instance, an optical modulator, is also possible. Production of highly-linear laser frequency chirps can lead to the achievement of substantially Fourier-limited range peaks, and the realization Cramer-Rao-limited distance measurement precisions (σ≈ΔR/√{square root over (SNR)}, where SNR is the RF power signal-to-noise ratio of the range measurement).

Existing optical measurement techniques and systems (e.g., existing LiDAR systems) may require larger complexity, ambiguous range, and/or greater processing time that may be available or desired in some applications.

SUMMARY

Examples of methods are described herein. An example method includes providing an interference signal from a frequency-modulated continuous-wave (FMCW) laser radar system, the interference signal based in part on a laser beam reflected from an object, digitizing the interference signal using a digitizer having a Nyquist frequency lower than an actual beat frequency of the interference signal to produce a digitized signal, the digitized signal consistent with multiple candidate beat frequencies, processing the digitized signal to select one of the multiple candidate beat frequencies corresponding to the actual beat frequency, and determining a distance to at least a portion of the object based on the beat frequency.

In some examples, the digitizer may include a first analog to digital converter (ADC). The digitized signal may include a first digitized signal corresponding to an output of the first ADC and the first digitized signal may be consistent with first multiple beat frequencies. An example method may further include digitizing the interference signal using a second ADC having a second Nyquist frequency to produce a second digitized signal, the second digitized signal consistent with a second set of multiple candidate beat frequencies, and the processing may include selecting a matching one from the first and second set of multiple candidate beat frequencies.

Examples of systems are described herein. An example system may include at least one chirped laser beam configured to be directed toward, and at least partially reflected by, an object to provide a reflected laser beam. The system may include a detector configured to combine the at least one chirped laser beam with the reflected laser beam to provide an interference signal. The system may include a first analog to digital converter configured to provide a first digital signal based on the interference signal, the first analog to digital converter having a first Nyquist frequency below a beat frequency of the interference signal. The system may include a second analog to digital converter configured to provide a second digital signal based on the interference signal, the second analog to digital converter having a second Nyquist frequency. The system may include at least one processor configured to determine a range to the object based on the first digital signal and the second digital signal.

In some examples, the first digital signal may be consistent with a first set of multiple candidate beat frequencies and the second digital signal may be consistent with a second set of multiple candidate beat frequencies. The processor may be configured to identify an actual beat frequency based on a candidate beat frequency included in both the first set of multiple candidate beat frequencies and the second set of multiple candidate beat frequencies.

Another example method may include chirping a first laser beam and a second laser beam in opposite directions to provide a first chirped beam and a second chirped beam, providing a first local oscillator beam based on the first chirped beam and a second local oscillator beam based on the second chirped beam, applying a frequency shift to the first chirped beam and the second chirped beam to provide a frequency shifted first chirped beam and a frequency shifted second chirped beam, directing the first chirped beam and the second chirped beam toward an object, receiving a first reflected beam corresponding to reflection of the first chirped beam from the object and a second reflected beam corresponding to a reflection of the second chirped beam from the object, generating a first interference signal between the first reflected beam and the first local oscillator beam and generating a second interference signal between the second reflected beam and the second local oscillator beam, and determining a distance to the object based on the first interference signal and the second interference signal.

Another example system may include at least one laser source, the at least one laser source configured to generate a first chirped laser and a second chirped laser, the first chirped laser and the second chirped laser chirped in opposite directions. The system may include a modulator configured to provide a shifted version of the first chirped laser and the second chirped laser for use as a first local oscillator and a second local oscillator. The system may include a combiner configured to combine the first local oscillator with a reflection of the first chirped laser from an object, and to combine the second local oscillator with a reflection of the second chirped laser from the object to generate a first interference signal and a second interference signal, respectively. The system may include a processor configured to determine a range to at least a portion of the object using the first interference signal and the second interference signal.

