Figure 1 shows a simplified block diagram of an improved optoelectronic system for locking the phase of one laser to that of another laser with an adjustable offset frequency specified by the user. In comparison with prior systems, this system exhibits higher performance (including higher stability) and is much easier to use. The system is based on a field-programmable gate array (FPGA) and operates almost entirely digitally; hence, it is easily adaptable to many different systems. The system achieves phase stability of less than a microcycle. It was developed to satisfy the phase-stability requirement for a planned spaceborne gravitational-wave-detecting heterodyne laser interferometer (LISA). The system has potential terrestrial utility in communications, lidar, and other applications.
The present system includes a fast phasemeter that is a companion to the microcycle-accurate one described in “High-Accuracy, High-Dynamic-Range Phase-Measurement System” (NPO-41927), NASA Tech Briefs, Vol. 31, No. 6 (June 2007), page 22. In the present system (as in the previously reported one), beams from the two lasers (here denoted the master and slave lasers) interfere on a photodiode. The heterodyne photodiode output is digitized and fed to the fast phasemeter, which produces suitably conditioned, low-latency analog control signals which lock the phase of the slave laser to that of the master laser. These control signals are used to drive a thermal and a piezoelectric transducer that adjust the frequency and phase of the slave-laser output.
The output of the photodiode is a heterodyne signal at the difference between the frequencies of the two lasers. (The difference is currently required to be less than 20 MHz due to the Nyquist limit of the current sampling rate. We foresee few problems in doubling this limit using current equipment.) Within the phasemeter, the photodiode-output signal is digitized to 15 bits at a sampling frequency of 40 MHz by use of the same analog-to-digital converter (ADC) as that of the previously reported phasemeter. The ADC output is passed to the FPGA, wherein the signal is demodulated using a digitally generated oscillator signal at the offset locking frequency specified by the user. The demodulated signal is low-pass filtered, decimated to a sample rate of 1 MHz, then filtered again. The decimated and filtered signal is converted to an analog output by a 1 MHz, 16-bit digital-to-analog converters. After a simple low-pass filter, these analog signals drive the thermal and piezoelectric transducers of the laser.
Although the system phase-locks the two lasers to within a microcycle, in the original application, there is an occasional need to analyze the performance of the phasemeter in the presence of noise in the difference between the phases and frequencies between the two lasers. This system includes a subsystem, based on a pseudorandom-number generator, that can add an adjustable amplitude phase noise characterized by a uniform, Gaussian, or 1/f distributions (where f denotes frequency). Figure 2 shows the performance of the phase-locking system, and also noise added by the pseudo-random noise generator to mimic that of free-running lasers.
This work was done by Daniel Shaddock and Brent Ware of Caltech for NASA’s Jet Propulsion Laboratory. NPO-44740
This Brief includes a Technical Support Package (TSP).
Fast Offset Laser Phase-Locking System
(reference NPO-44740) is currently available for download from the TSP library.
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Overview
The document outlines NASA's Fast Offset Laser Phase-Locking System, designated as NPO-44740, developed at the Jet Propulsion Laboratory (JPL). This innovative system addresses the challenges faced in the Laser Interferometer Space Antenna (LISA) mission, which involves three spacecraft positioned 5 million kilometers apart in an Earth-trailing orbit. Each spacecraft emits a laser signal that must be accurately interfered with the local laser to measure gravitational waves. However, the laser frequency noise can obscure these measurements, necessitating a robust phase-locking mechanism.
Traditional analog phase-locked loops struggle to maintain the required microcycle phase stability over extended periods due to temperature variations affecting physical components. The Fast Offset Laser Phase-Locking System overcomes these limitations by implementing a digital phase-locked loop. This system digitizes the signal at high speeds, allowing for most signal processing to be conducted digitally. Digital filters are inherently stable, exhibiting zero temperature dependence, which enhances the system's performance and reliability.
The digital phase-locked loop enables the phase-locking of two lasers with exceptional precision, achieving stability down to the microcycle phase level. This capability is crucial for LISA, which has stringent requirements for phase stability. The system is designed to be programmable and flexible, utilizing a field programmable gate array (FPGA) that allows for easy adaptation to various applications.
Additionally, the system can simulate noisy environments by adding known phase noise, which can be uniform, Gaussian, or 1/f, with adjustable amplitude. This feature is particularly useful for testing and validating the system's performance under different conditions.
The document emphasizes the novelty and advantages of this phase-locking system compared to previous solutions, which were often cumbersome and difficult to use. The Fast Offset Laser Phase-Locking System represents a significant advancement in laser technology, providing higher performance, stability, and ease of use, making it a vital component for future gravitational wave detection missions.
For further inquiries or detailed information, the document provides contact details for the Innovative Technology Assets Management team at JPL.
