The next generation of Gravity Recovery and Climate Experiment (GRACE)-like dual-satellite geodesy missions proposals will rely on inter-spacecraft laser interferometry as the primary instrument to recover geodesy signals. Laser frequency stability is one of the main limits of this measurement and is important at two distinct timescales: short timescales over 10-1000 seconds to measure the local gravity below the satellites, and at the month to year timescales, where the subsequent gravity measurements are compared to indicate loss or gain of mass (or water and ice) over that period. This paper demonstrates a simple phase modulation scheme to directly measure laser frequency change over long timescales by comparing an on-board Ultra-Stable Oscillator (USO) clocked frequency reference to the Free Spectral Range (FSR) of the on-board optical cavity. By recording the fractional frequency variations the scale correction factor may be computed for a laser locked to a known longitudinal mode of the optical cavity. The experimental results demonstrate a fractional absolute laser frequency stability at the 10 ppb level (10⁻⁸) at time scales greater than 10 000 seconds, likely sufficient for next generation mission requirements.

中文翻译：

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用于下一代大地测量任务的来自 ULE 腔的绝对频率读数

下一代类似重力恢复和气候实验 (GRACE) 的双卫星大地测量任务提案将依赖航天器间激光干涉测量作为恢复大地测量信号的主要仪器。激光频率稳定性是这种测量的主要限制之一，在两个不同的时间尺度上很重要：超过 10-1000 秒的短时间尺度来测量卫星下方的局部重力，以及在月到年的时间尺度上，随后的重力测量是相比，表示在那段时间内质量（或水和冰）的损失或增加。本文演示了一种简单的相位调制方案，通过将机载超稳定振荡器 (USO) 时钟频率参考与机载光学腔的自由光谱范围 (FSR) 进行比较，直接测量长时间尺度上的激光频率变化。通过记录分数频率变化，可以为锁定到光腔的已知纵模的激光器计算标度校正因子。实验结果表明，在 10 ppb 水平 (10⁻⁸ ) 在大于 10 000 秒的时间尺度上，可能足以满足下一代任务要求。