This project’s goal was to simplify laser frequency stabilization. A simpler system will have many benefits, including reduction of power consumption, complexity, volume, mass, and risk of failure. To implement the Pound-Drever-Hall (PDH) technique requires both RF modulation and demodulation electronics, including an electro-optic modulator, a photoreceiver of sufficient bandwidth to detect the RF modulation fields, demodulation electronics of sufficient bandwidth, and an RF function generator. For a space mission, this equipment can be costly and power-hungry, in addition to the difficulty of being rated to operate in the harsh space environment.
The need for laser frequency stabilization will occur in many planned missions employing multiple lasers on multiple spacecraft (SIM, LISA, GRACE Follow-on, GRACE C). In addition, many laboratory measurements require laser frequency stabilization. This software-based system obviates the need for RF components (modulator, detector, frequency/function generator, and associated electronics), both in the laboratory and on a spacecraft, to perform laser frequency stabilization in a digitally controlled system.
The code-based (digital) technique was demonstrated for laser frequency stabilization to an optical reference cavity without requiring any RF electronics or RF equipment. The process is coded using LabVIEW and employs field programmable gate arrays (FPGAs) for digital input and output from the photoreceiver and laser, respectively. Instead of employing an external phase-modulator driven by a function generator, the digital code directly modulates the laser’s piezoelectric transducer (PZT).
This work has shown that high laser frequency stability performance can be achieved using optical cavity references, without the need for the traditional PDH RF electronics. This was demonstrated using digital software codes and control, significantly simplifying the analog electronics. This technique reduces mass and power associated with the RF electronics. This software technique is implemented via hardware that will already exist onboard the spacecraft. This also reduces complexity, power consumption, volume, and risk of failure.
This work was done by Glenn DeVine, Brent Ware, Kirk McKenzie, Robert J. Thompson, William M. Klipstein, William M. Folkner, and Robert E. Spero of Caltech for NASA’s Jet Propulsion Laboratory.
The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at
This Brief includes a Technical Support Package (TSP).
Digital Laser Frequency Stabilization via Cavity Locking Employing Low-Frequency Direct Modulation
(reference NPO-48530) is currently available for download from the TSP library.
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Overview
The document titled "Digital Laser Frequency Stabilization via Cavity Locking Employing Low-Frequency Direct Modulation" discusses advancements in laser frequency stabilization techniques, particularly relevant for precision metrology in space missions such as LISA (Laser Interferometer Space Antenna) and GRACE-FO (Gravity Recovery And Climate Explorer – Follow-On). These missions require highly accurate distance measurements, achievable through lasers that can provide precision 1000 times greater than traditional microwave systems.
A significant challenge in utilizing lasers for such measurements is their inherent frequency noise. To address this, the document outlines the state-of-the-art method of laser frequency stabilization using an optical reference cavity. This cavity consists of two highly reflective mirrors attached to a solid spacer made from ultra-stable materials, such as ultra-low expansion glass. This setup provides a stable length reference that is more reliable than the laser frequency itself. The laser probes the cavity, and the difference between the laser frequency and the reference cavity length is used to stabilize the laser frequency through feedback mechanisms.
The document highlights the widely used Pound-Drever-Hall (PDH) technique, which employs RF modulation sidebands to measure the frequency difference. However, this method requires additional hardware, increasing complexity, weight, and power consumption, which can be a drawback for space missions.
In response to these challenges, the authors propose a digital laser frequency stabilization technique that minimizes the need for extra hardware. This approach leverages existing onboard spacecraft systems, reducing complexity, power consumption, and the risk of failure. The technique is applicable to various laser frequency stabilization systems, whether in laboratory settings, airborne, or space environments.
The research was conducted at NASA's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration, emphasizing its significance in advancing aerospace technology. The document serves as a technical support package, providing insights into the innovative methods that can enhance the performance of laser systems in demanding applications, ultimately contributing to the success of future space missions.
