A frequency-modulated continuous-wave (FMCW) lidar is demonstrated with heterodyne detection. The lidar transmitter utilizes an electro-optic I/Q modulator for the first time to generate carrier-suppressed and frequency-shifted FM modulation. This eliminates the need for an acousto-optic frequency shifter commonly used in heterodyne lidar transmitters. It also allows the use of a much wider modulation bandwidth to improve the range resolution. The capability of complex optical field modulation of the I/Q modulator provides an additional degree of freedom compared with an intensity modulator, which will benefit future lidar applications.

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Frequency-modulated continuous-wave lidar using I/Q modulator for simplified heterodyne detection.

A coherent lidar system based on the measurement of complex optical field is demonstrated for the first time. An electro-optic in-phase/quadrature (I/Q) modulator is used in the lidar transmitter to realize carrier-suppressed complex optical field modulation in which the positive and the negative optical sidebands can carry independent modulation waveforms. A fiber-optic 90° hybrid is used in the lidar receiver for coherent heterodyne detection and to recover the complex optical field. By loading a constant modulation frequency on the lower optical sideband and a wideband linear frequency chirp on the upper sideband, vector velocity and target distance can be measured independently. The wide modulation bandwidth of this lidar system also enabled unprecedented range resolution and the capability of measuring high velocity unambiguously.

https://kuscholarworks.ku.edu/bitstream/handle/1808/11546/Hui_Complex%20optical%20field%20lidar%20system.pdf;sequence=1

Complex-optical-field lidar system for range and vector velocity measurement

A novel coherent interferometric dual frequency laser radar that merges the concept of laser and radio detection and ranging (ladar and radar, respectively), for both range and velocity measurement, is presented and experimentally demonstrated. The innovative architecture combines the broadband tunability of dual-wavelength optical sources, enabling a dynamic trade-off between precision and robustness in Doppler estimation, with the high stability of low frequency RF sources for the interferometric measure of the target range with extreme accuracy. The possibility to easily reconfigure the employed frequencies, allows to change the Doppler resolution, as well as the range ambiguity and precision, to dynamically adapt the system to reliably operate in different environments. Moreover, the coherent detection allows to enhance the signal to noise ratio reaching excellent performances also with low level of received power. The laboratory characterization provides an estimation of the system performances, in terms of resolution and sensitivity, as well as the indoor demonstration with targets of opportunity proves the effectiveness of the proposed architecture to operate in real scenarios.

Coherent Interferometric Dual-Frequency Laser Radar for Precise Range/Doppler Measurement

We propose a novel architecture for implementing a dual-frequency lidar (DFL) exploiting differential Doppler shift measurement The two frequency tones, needed for target velocity measurements, are selected from the spectrum of a mode-locked laser operating in the C-band The tones’ separation is easily controlled by using a programmable wavelength selective switch, thus allowing for a dynamic trade-off among robustness to atmospheric turbulence and sensitivity Speed measurements for different tone separations equal to 10, 40, 80, and 160 GHz are demonstrated, proving the system’s capability of working in different configurations Thanks to the acquisition system based on an analog-to-digital converter and digital-signal processing, real-time velocity measurements are demonstrated The MLL-based proposed architecture enables the integration of the DFL with a photonic-based radar that exploits the same laser for generating and receiving radio-frequency signal with high performance, thus allowing for simultaneous or complementary target observations by exploiting the advantages of both radar and lidar

Frequency-agile dual-frequency lidar for integrated coherent radar-lidar architectures.

A new architecture for an integrated fully coherent radar–lidar system based on a single mode-locked laser is proposed and demonstrated. The lidar exploits a multi-frequency optical signal with tunable tones separation allowing a dynamic tradeoff among robustness and sensitivity of measurements. The radar that is based on photonics technologies employs the mode-locked laser for generating the radio frequency signals in the X-band and Ku-band, simultaneously with the lidar. Velocity measurements for different tones separation are demonstrated with good agreement among the values measured with the lidar and radar.

Multi-Frequency Lidar/Radar Integrated System for Robust and Flexible Doppler Measurements

The principles of coherent laser spectroscopy are briefly reviewed in which a signal beam is heterodyned with an optical local oscillator beam. The frequency and single mode selectivity this provides is shown to be very advantageous. This is illustrated by recent work on the properties of different classes of light field, including the fractal character of Gaussian-Lorentzian light, and by several aspects of laser physics. In equivalent practical applications to remote sensing in the atmosphere, coherent Doppler lidar provides a powerful technique. This is illustrated by measurements of wind fields and aircraft wake vortices. Airborne equipments have been used for measurements of true airspeed, wind shear warning and atmospheric backscatter levels. Future space-borne lidars could potentially measure the global wind field—of great benefit for numerical weather forecasting and climate studies.

http://iopscience.iop.org/article/10.1238/Physica.Topical.078a00073/pdf

J. M. Vaughan

Papers

Coherent Laser Spectroscopy and Doppler Lidar Sensing in the Atmosphere