A method reduces sensitivity to noise in a signal from a laser heterodyne interferometer. The phase-locked loop (PLL) removes glitches that occur in a zero-crossing detector's output [that can happen if the signal-to-noise ratio (SNR) of the heterodyne signal is low] by the use of an internal oscillator that produces a square-wave signal at a frequency that is inherently close to the heterodyne frequency.
It also contains phase-locking circuits that lock the phase of the oscillator to the output of the zero-crossing detector. Because the PLL output is an oscillator signal, it is glitch-free. This enables the ability to make accurate phase measurements in spite of low SNR, creates an immunity to phase error caused by shifts in the heterodyne frequency (i.e. if the target moves causing Doppler shift), and maintains a valid phase even when the signal drops out for brief periods of time, such as when the laser is blocked by a stray object.
This work was done by Frank Loya and Peter Halverson of Caltech for NASA’s Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
NASA Management Office–JPL.
Refer to NPO-40080.
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Laser Metrology Heterodyne Phase- Locked Loop
(reference NPO-40080) is currently available for download from the TSP library.
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Overview
The document outlines advancements made by NASA's Jet Propulsion Laboratory (JPL) in the field of laser metrology, specifically focusing on the use of a phase-locked loop (PLL) to enhance the performance of laser heterodyne interferometers. These devices are crucial for precise distance measurements to reflective targets, such as mirrors, and are widely used in various applications, including aerospace.
The primary challenge addressed in this work is the sensitivity of the JPL's high-resolution phasemeter to glitches caused by low signal-to-noise ratio (SNR) in the heterodyne signal. Low SNR can occur due to factors such as low laser power, long measurement distances, or low target reflectivity. Traditional methods, such as band-pass filtering, have limitations, particularly when the heterodyne frequency varies over time, which can introduce phase errors.
To overcome these challenges, the document describes the integration of a PLL between the zero-crossing detector (ZCD) and the phasemeter. The ZCD converts the sine wave heterodyne signal into a square wave, which the phasemeter can process. However, low SNR can lead to glitches in the ZCD output, hindering accurate phase measurements. The PLL effectively mitigates these glitches by locking the phase of an internal oscillator to the output of the ZCD, resulting in a glitch-free square wave signal.
The advantages of this PLL approach include the ability to make accurate phase measurements even in low SNR conditions, immunity to phase errors caused by Doppler shifts when the target moves, and the capability to maintain valid phase readings during brief signal dropouts. However, there are some disadvantages, such as the potential for the PLL to fail to lock to the heterodyne signal, temporary phase errors during rapid frequency changes, and the risk of losing lock if the heterodyne frequency changes too quickly.
Overall, this document highlights the innovative use of PLL technology to improve the precision and reliability of laser heterodyne interferometry, addressing key challenges in the field and paving the way for enhanced measurement capabilities in various applications. The work represents a significant step forward in laser metrology, with implications for both scientific research and commercial technology.
