NASA’s Langley Research Center has made a breakthrough improvement in laser frequency modulation. Frequency modulation technology has been used for surface mapping and measurement in sonar, radar, and time-of-flight laser technologies for decades. Although adequate, the accuracy of distance measurements made by these technologies can be improved by using a high-frequency triangular-waveform laser instead of a sine waveform or lower-frequency radio or microwaves. This new system generates a triangular modulation waveform with improved linearity that makes possible precision laser radar [light detection and ranging (lidar)] for a variety of applications.
For decades, frequency modulation has been used to generate chirps, the signals produced and interpreted by sonar and radar systems. Traditionally, a radio or microwave signal is transmitted toward the target and reflected back to a detector, which records the time elapsed and calculates the target’s distance. Reflected signals can be heterodyned (combined) with output signals to determine the Doppler frequency shift and the target velocity. Accuracy of these systems can be enhanced by increasing the bandwidth of the chirp, but noise generated during heterodyning at high frequencies decreases the signal-to-noise ratio, increasing measurement error.
Previous attempts at laser frequency modulation that relied on adjusting the laser cavity length have resulted in only sine wave or imperfect triangle waveforms. Heterodyning of imperfect, nonlinear waveforms or sine waveforms will significantly degrade the effective signal-to-noise ratio, making such systems impractical. In contrast, the current technology produces a single, high-frequency laser that is passed to an electro-optical modulator, which generates a series of harmonics. This range of frequencies is then passed through a band-pass optical filter so the desired harmonic frequency can be isolated and directed toward the target. By modulating the electrical signal applied to the electro-optical modulator, a near-perfect triangular waveform laser beam can be produced. Transmission and detection of this highly linear triangular waveform facilitates optical heterodyning for the calculation of precise frequency and phase shifts between the output and reflected signals with a high signal-to-noise ratio. By combining this information with the time elapsed, the location and velocity of the target can be determined to within 1 mm or 1 mm/s.
Users have the ability to measure air velocity, ground velocity, target distance and velocity, aircraft altitude, angle of attack, and atmospheric wind vector in one system. It is more accurate and reliable than current pitot-tube aircraft instrumentation that can ice up and requires frequent calibration. In addition, there is an order-of-magnitude improvement in accuracy over time-of-flight laser pulse systems, and multiple orders of magnitude improvement as compared to radar systems for distance and velocity measurements.
Potential applications include spacecraft landing and docking; planet topography measurement; precision alignment of large structures in manufacturing and construction; movement accuracy and maneuverability in confined spaces for robotics; replacement of pitot-static instrumentation systems for air velocity, ground velocity, altitude, and attitude measurements; target ranging and 3D visualization of structures and surfaces in aerospace systems; movement detection and target visualization for surveillance and security; ground and target imaging for military systems; and molecule identification for spectroscopy.