Optimal Phase Lock at Femtowatt Power Levels for Coherent Optical Deep-Space Transponder

We report on results of the first tests of an optical phase-lock loop (PLL) with the capabil ity to recover an optical carrier at powers below one picowatt (pW), as required for a deepspace coherent optical transponder. In such an application, a limiting phase variation is due to instability in the (optical) frequency standard that provides a phase reference at the return of the transponded signal, possibly many minutes later. We present results showing a PLL phase-slip rate below one cycle slip per second at powers as low as 40 femtowatts (fW). This phase-slip rate corresponds to a frequency stability of d f /f . 1 × 10 –14 / x , a value better than any frequency standard available today for measuring times equal to a typical two-way delay between Earth and Mars, and the 1100 fW required power allows application at Mars’ farthest distance from Earth with a reasonable transmitter power level (P 15 W). Parameters for a second-order PLL were optimized for the laser-noise and shotnoise levels by use of simulation software. Good agreement was obtained between laboratory measurements and the simulation results. We also report on measurements of the phase noise of the fiber lasers used for the test, analysis of the limitations on PLL performance at low optical power implied by this laser noise level, the results of demonstrations of Doppler signals at fW power levels, and transponder architectures designed to take advantage of this capability to achieve micron-level ranging accuracy with very short measuring times.