TECHNICAL FIELD

This disclosure relates generally to light detection and ranging systems. More particularly, this disclosure relates to light detection and ranging systems that utilize optical interferometry devices and methods to measure distance and speed.

BACKGROUND

Light detection and ranging (LiDAR) is analogous to radio detection and ranging (Radar) in that LIDAR uses optical waves to determine the range, angle, and velocity of objects. LiDAR utilizes differences in laser light return times and wavelengths that can be used to make digital 3-D representations of the target and has been broadly used in terrestrial, airborne, and mobile applications. A LiDAR instrument consists of one or more laser emitters, optics, a scanner, a photodetector, and a signal processor. The one or more laser emitters generate a coherent light beam transferred through a set of optics to a scanner to be transmitted to an object for determining the distance to the object or the object's velocity. In the case of three-dimensional (3D) scanning, the physical features are determined. The photodetector receives the coherent light reflected from the object and converts the coherent light to electrical signals that are processed to determine the object's distance. The emitter will generate the coherent light as pulses. The signal processor records the time of the transmitted pulse, and the time of the reception of the reflection of the coherent light is recorded. The distance is the difference between the transmitted time and the received time divided by two and multiplied by the speed of light.

Amplitude modulation continuous wave (AMCW) LiDAR is a form of LIDAR that is phase-based. Unlike direct pulse detection, the phase-based LIDAR emits a continuous laser signal. It modulates the laser emission amplitude with a high-speed radiofrequency (RF) signal to encode the output optical signal. The phase difference between the emitted signal and the reflected signal is detected for ranging. The phase shift of a sinusoidally-modulated continuous laser waveform can be used to infer the distance to an object.

Frequency modulated continuous wave (FMCW) LiDAR is similar to AMCW LiDAR, but the modulation and demodulation are performed optically rather than electrically. FMCW LiDAR uses a wavelength-tuning light source and an interferometer to measure the object's distance with good sensitivity. “Comb-Calibrated Frequency-Modulated Continuous-wave Lidar, “Y. Xie et al., 2020 IEEE 7th International Workshop on Metrology for AeroSpace (MetroAeroSpace), Pisa, Italy, 2020, pp. 372-376, Found Feb. 15, 2021 at URL: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9160234&isnumber=9159966 describes FMCW LIDAR as very suitable for absolute distance measurement. The FMCW laser's frequency is linearly modulated by a carrier signal to measure the laser round-trip flight time accurately. The flight time can be calculated with high precision by detecting the beat frequency signal between the returned laser and the emitted laser. High-precision distance measurement can be achieved.

SUMMARY

An object of this disclosure is to provide a time-of-interference (TOI) light detection and ranging (LiDAR) system that is based on time-frequency domain reflectometry. The TOI LIDAR system uses a time-to-digital converter or a data acquisition system to record the time delay of an interference signal or time-of-interference (TOI).

Another object of this disclosure is to provide a TOI LiDAR system based on small wavelength modulation of the coherent light source. The output wavelength is determined by the operating current or operating temperature of the coherent light source.

To accomplish at least one of these objects, a TOI LIDAR system has a coherent light source connected to a modulating controller. The modulating controller is configured to generate a pulsed wavelength control signal that is transferred to the coherent light source. The pulsed wavelength control signal may be a current modulation signal or a laser ambient temperature adjustment signal. The pulsed wavelength control signal modulates the coherent light source to generate a pulsed wavelength-modulated coherent light emission.

The pulsed wavelength-modulated coherent light emission is an input to an interferometer. The interferometer is configured for partitioning the pulsed wavelength-modulated coherent light emission into a sampling portion and a reference portion. The pulsed wavelength-modulated coherent light emission's sampling portion is arranged to impinge upon an object to be measured. The reference portion of the pulsed wavelength-modulated coherent light emission is arranged to provide a reference basis for determining the distance from the TOI LIDAR system to the object. The interferometer further is configured for transferring the pulsed wavelength-modulated coherent light to a scanner. The scanner is configured to physically transfer the pulsed wavelength-modulated coherent light's first portion to the object and scan the object's surface with the pulsed wavelength-modulated coherent light. The scanner is further configured to receive a portion of the pulsed wavelength-modulated coherent light back-reflected from the object. The back-reflected pulsed wavelength-modulated coherent light is transferred from the scanner to the interferometer and then coupled with the reference portion of the pulsed wavelength-modulated coherent light to form an optical interference light signal.

The TOI LIDAR system has a photodetector array configured to convert the optical interference signal to electrical interference signal. In various embodiments, the photodetector is configured as a polarization-diversity balanced amplified detector. The photodetector has at least one power monitor to measure the input power level to the photodetector. The power monitor output provides a modulated power level with a time delay associated with the object's distance.

The TOI LIDAR system has a signal processor configured to receive the electrical interference signal and convert the electrical interference signal to digital data representing the electrical interference signal amplitude as the digital data. The signal processor is configured to generate a displayed imaging range based on the distance from the target. The displayed imaging range is calculated by a computer system programmed to calculate the time delay determined by the optical interference signal.

The modulating controller is configured to generate a wavelength modulation control signal to modulate the coherent light source by controlling the narrow coherent light source's driving current, the temperature of the narrow bandwidth light source, or adjusting the phase of the light emitted from the light source. In other embodiments, the modulating controller will generate a pulsed phase control signal for generating interference when there is a time delay between light in the sample and reference arms of the interferometer.

In various embodiments, the interferometer includes a polarization controller used to adjust the polarization states of the coherent light emission from the light source and maximize the amplitude of the optical interference signal or interference electrical signal. The interferometer has a first coupler that receives the pulsed wavelength-modulated coherent light from the polarization controller. The coupler divides the pulsed wavelength-modulated coherent light. A first portion of the pulsed wavelength-modulated coherent light is fed into at least one sample arm. A second portion of the pulsed wavelength-modulated coherent light is fed to a reference arm. The interferometer has a circulator connected to receive the first portion of the pulsed wavelength-modulated coherent light from the at least one sample arm. The circulator is configured such that the pulsed wavelength-modulated coherent light from the sample arm enters the circulator and exits from the next port. Generally, the next port is in a clockwise direction to direct the pulsed wavelength-modulated coherent light to the scanner. The scanner is configured to physically transfer the sample pulsed wavelength-modulated coherent light to scan the objects. The sampled pulsed wavelength-modulated coherent light is back-reflected from the object for making ranging measurements to the scanner and transferred to the circulator within the interferometer. The back-reflected pulsed wavelength-modulated coherent light is then transmitted from the circulator to a second coupler.

The interferometer's reference arm has a length greater than two times the sampling arm's length. The second portion of the pulsed wavelength-modulated coherent light in the reference arm is applied to the second coupler. The second portion of the pulsed wavelength-modulated coherent light transported in the reference arm is coupled with the collected back-reflected pulsed wavelength-modulated light to form an optical interference light signal. The optical interference light signal exits the second coupler to enter a photodetector array.

The reference arm's optical path length is longer than the sample arm's optical path length by more than two times the system's maximum ranging depth. The maximum frequency of the optical interference signal corresponds to the minimum ranging depth of the system.

The maximum frequency of the optical interference signal corresponds to the minimum ranging depth of the TOI LIDAR system. It is greater than a Nyquist sampling frequency of the digitizer in the data acquisition and signal processor. The minimum frequency of the optical interference signal corresponds to the maximum ranging depth of the TOI LIDAR system. The time delay of the detected optical interference is measured at the falling edge of the optical interference signal's envelope.

