Laser link acquisition demonstration for the GRACE Follow-On mission

Multi-target free-space laser communication system based on a rotating double prism with an RF signal beacon

Traditional free-space laser communication systems use beacon and signal lights for target detection and alignment. However, these approaches are inaccurate owing to signal dispersion errors. To overcome this difficulty, we propose a new method using transient radio frequency (RF) signals to achieve highly accurate target detection and alignment. To validate the feasibility of our proposed method, we built an experimental multi-target space-laser communication system based on a rotating double prism and applied it to achieve multi-target space-laser communication. The results demonstrate the efficiency of the proposed method to capture multi-target positions in the field of view using wireless RF signals and a rotating double prism. In addition, we show that the system is capable of rapid scanning and accurate pointing as well as establishing a one-way stable communication with multiple targets. When the target is 36 cm away, the pointing accuracy of the system motor is less than 0.8°, the pointing time is 1.2 s, and the average pointing lateral error is 0.666 mm.

Shot-noise-limit performance of a weak-light phase readout system for intersatellite heterodyne interferometry

A laser interferometer will be used in the spaceborne gravitational-wave detection missions to measure the inter-satellite optical pathlength variations. The phase readout system of the interferometer needs to be carefully designed and tested to accomplish a shot-noise-limited detection performance under the situation of pico-Watt level received lights. In this work, a scheme based on dual-tone acousto-optic diffraction is presented to verify the performance of the weak-light phase readout system. By optimizing the parameters of the photoreceiver and the local strong-light power, the signal-to-noise ratio of the beat-note signal is enhanced. Thanks to the scheme's common-mode noise rejections for the laser frequency noise, and the optical-path noise, etc., the differential phase noise has achieved a performance of 2×10^-4 rad/Hz^1/2, which is dominated by the weak-light (∼13 pW) shot noise above the frequencies of 2 mHz.

Analysis of performance and robustness against jitter of various search methods for acquiring optical links in space

We discuss various methods for acquiring optical links in space using a dedicated acquisition sensor. Statistical models are developed and simple analytical equations derived that compare the performance between a single- and dual-spiral scan approach as well as between sequential and parallel acquisition of link chains. Simple derived analytical equations allow relating essential search parameters such as track width, variance of the uncertainty distribution, capture radius, and scan speed to the probabilities of acquiring the links within a specific time. We also assess the probability of failing to acquire a link due to beam jitter and derive a simple analytical model that allows determining the maximum tolerable jitter for a given beam overlap and required probability of success. All results are validated by Monte Carlo simulations and applied to the concrete example of the GRACE FO mission.

Laser acquisition experimental demonstration for space gravitational wave detection missions

The laser acquisition-pointing technique is one of the most important techniques for space gravitational wave detection missions, like the Taiji program and the LISA (Laser Interferometer Space Antenna) program. The laser acquisition system suppresses the laser deviation angle to 1 µrad at the receiving aperture. Corresponding to 80 times of telescope magnification, the acquisition accuracy should reach 80 µrad at the acquisition camera. In order to verify the feasibility of the laser acquisition scheme, a laser acquisition ground simulation experimental system is designed and constructed. The experimental system simulates the actual acquisition process of the Taiji from three aspects: optical path, acquisition accuracy and acquisition scanning process. In the experiment, the coupling between the laser acquisition system and the laser pointing system is considered by introducing the DWS (Differential Wave-front Sensing) technique to calibrate the reference position of the acquisition camera and read out the acquisition precision. Due to limited beam propagation distance in the ground experiment, the in-flat top properties of the transmitting beam will greatly affect the acquisition precision. Based on the analysis of the influence, an improved acquisition ground simulation scheme is introduced. The experimental results indicate that the experimental system can achieve the acquisition accuracy of sub-10 µrad magnitude at the acquisition camera. The experimental system realizes methodological demonstration of the acquisition scheme. The results offer the experimental foundation and theoretical basis for the acquisition system of the Taiji/LISA program.

Fast beam steering with an optical phased array

Optical phased arrays (OPAs) are devices that use the coherence of light to control the interference pattern in the far field, which enables them to steer a laser beam with no moving parts. As such, OPAs have potential applications in laser communications, target acquisition and tracking, metrology, and directed energy. In this Letter, we present a control architecture for an actively phase-locked OPA, capable of steering a laser beam at speeds limited by the actuation bandwidth of electro-optic modulators. The system achieved an output phase stability of λ/770 and steering speeds up to 1 MHz. The digital control architecture can be extended to GHz steering speeds, is readily scalable to hundreds of emitters, and is compatible with high-power arrays.

Shipborne Acquisition, Tracking, and Pointing Experimental Verifications Towards Satellite-to-Sea Laser Communication

Acquisition, tracking, and pointing (ATP) is a key technology in free space laser communication that has a characteristically high precision. In this paper, we report the acquisition and tracking of low-Earth-orbit satellites using shipborne ATP and verify the feasibility of establishing optical links between laser communication satellites and ships in the future. In particular, we developed a shipborne ATP system for satellite-to-sea applications in laser communications. We also designed an acquisition strategy for satellite-to-sea laser communication. In addition, a method was proposed for improving shipborne ATP pointing error. We tracked some stars at sea, achieving a pointing accuracy of less than 180μrad.We then acquired and tracked some low-Earth-orbit satellites at sea, achieving a tracking accuracy of about 20μrad. The results achieved in this work experimentally demonstrate the feasibility of ATP in satellite-to-sea laser communications.

In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1  nm/sqrt[Hz] at Fourier frequencies above 100 mHz.

Line of sight calibration for the laser ranging interferometer on-board the GRACE Follow-On mission: on-ground experimental validation

The laser ranging interferometer (LRI) on board of the GRACE follow-on spacecraft, launched in May 2018, is the first laser interferometer to perform an inter-satellite range measurement. It is designed for ranging noise levels of 80 nm Hz^-1/2 for frequencies above 20 mHz, i.e., about a ten-fold improvement with respect to the GRACE follow-on main microwave ranging instrument. One of the most critical steps during the commissioning phase of the instrument is the so-called initial line of sight calibration procedure (or initial acquisition). This process is required to quantify large uncertainties with respect to laser beam pointing angles and laser frequency, which must be known to establish the interferometer link. It is a nine hour scan of five degrees of freedom, which all need to match simultaneously at least once. Here we report on laboratory tests to further validate the calibration procedure using a mock-up LRI and a set-up, the so-called laser link simulator, that creates conditions similar to those with ~220 km distance between the SC. The experiments presented here made use of LRI-like hardware and software and were carried out recreating critical conditions such as received laser powers on the pico-Watt level and their dependence on the SC misalignments, flat-top beams as receiving beams and Doppler frequency shifts. Several configurations were tested, including a full line of sight calibration with angular scans in both mock-up SC and frequency scan in one of the lasers. Results are well in agreement with the expectations and confirm, well before the LRI commissioning phase, the robustness of the procedure under realistic conditions, which had not yet been fully tested experimentally.

Inter-satellite laser link acquisition with dual-way scanning for Space Advanced Gravity Measurements mission

Laser link acquisition is a key technology for inter-satellite laser ranging and laser communication. In this paper, we present an acquisition scheme based on the differential power sensing method with dual-way scanning, which will be used in the next-generation gravity measurement mission proposed in China, called Space Advanced Gravity Measurements (SAGM). In this scheme, the laser beams emitted from two satellites are power-modulated at different frequencies to enable the signals of the two beams to be measured distinguishably, and their corresponding pointing angles are determined by using the differential power sensing method. As the master laser beam and the slave laser beam are decoupled, the dual-way scanning method, in which the laser beams of both the master and the slave satellites scan uncertainty cones simultaneously and independently, can be used, instead of the commonly used single-way scanning method, in which the laser beam of one satellite scans and that of the other one stares. Therefore, the acquisition time is reduced significantly. Numerical simulation and experiments of the acquisition process are performed using the design parameters of the SAGM mission. The results show that the average acquisition time is less than 10 s for a scanning range of 1-mrad radius with a success rate of more than 99%.

Interspacecraft link simulator for the laser ranging interferometer onboard GRACE Follow-On

Link acquisition strategies are key aspects for interspacecraft laser interferometers. We present an optical fiber-based setup able to simulate the interspacecraft link for the laser ranging interferometer (LRI) on gravity recovery and climate experiment Follow-On. It allows one to accurately recreate the far-field intensity profile depending on the mispointing between the spacecraft, Doppler shifts, and spacecraft attitude jitter. Furthermore, it can be used in late integration stages of the mission, since no physical contact with the spacecraft is required. The setup can also be easily adapted to other similar missions and different acquisition algorithms.

A phasemeter concept for space applications that integrates an autonomous signal acquisition stage based on the discrete wavelet transform

We describe a phasemeter designed to autonomously acquire and track a heterodyne signal with low signal-to-noise ratio in a frequency band that spans from 1 MHz to 25 MHz. The background driving some of the design criterions of the phasemeter comes from studies on future space mission concepts such as orbiting gravitational wave observatories and next generation geodesy missions which all rely on tracking phasemeters in order to meet their mission goal. The phasemeter has been implemented within a field programmable gate array trying to minimize the requirement of computational resources and its performance has been tested using signal generators. Laboratory test has shown that the phasemeter is capable of locking to an input signal in less than half a millisecond, while its phase measurement accuracy is in the micro-radian range for measurement frequencies that span from mHz to Hz.

Predictive filtering-based fast reacquisition approach for space-borne acquisition, tracking, and pointing systems

We propose a novel approach for recapturing targets as quickly as possible after the space-borne ATP (Acquisition, Tracking, and Pointing) system target loss in free-space laser communication. The approach uses past angular information to predict the target trajectory. An optimized finite memory filter is designed as the prediction algorithm. To obtain optimal prediction performance, the designed filter determines the filter parameters of different curve parts during offline training, and the parameters are adjusted according to the curve characteristics during predictive filtering. Simulation results indicate prediction accuracy above 0.1 degrees within 5 seconds. An experimental system was constructed in the lab to simulate the reacquisition process using a dynamic target and a real ATP system. Experimental results show that, compared with the classical orbit prediction method, this approach can effectively yield shorter acquisition times.

Weak-light phase tracking with a low cycle slip rate

The Gravity Recovery and Climate Experiment Follow-On mission will use a phase-locked loop to track changes in the phase of an optical signal that has been transmitted hundreds of kilometers between two spacecraft. Beam diffraction significantly reduces the received signal power, making it difficult to track, as the phase-locked loop is more susceptible to cycle slips. The lowest reported weak-light phase locking is at 40 fW with a cycle slip rate of 1 cycle per second. By selecting a phase-locked loop bandwidth that minimized the signal variance due to shot noise and laser phase fluctuations, a 30 fW signal has been tracked with a cycle slip rate less than 0.01 cycles per second. This is tracking at a power 25% lower with a 100-fold improvement in the cycle slip rate. This capability will enable a new class of missions, opening up new opportunities for space-based interferometry.