Lunar Laser Ranging with High-Power CW Lasers

We present a comprehensive analysis of high-power continuous-wave (CW) lunar laser ranging (LLR) systems designed to achieve sub-millimeter Earth-Moon range precision. Using a 1 kW laser at 1064nm paired with a 1m-aperture telescope, we construct a detailed link budget that ac- counts for beam divergence, atmospheric transmission, retroreflector efficiency, and high-sensitivity single-photon detection. Key challenges include shot-noise limitations under multi-second coherent integration, atmospheric turbulence introducing path fluctuations of 300–500 μm, mechanical and thermal drifts in the optical system, and oscillator stability at the 10^−13–10^−14 level. By maintaining sub-0.01◦C thermal stability and leveraging ultra-stable frequency references, photon fluxes of ∼ 10^4 s−1 are achievable, reducing statistical noise to sub-millimeter levels. We further explore differential LLR, alternating between retroreflectors separated by ∼ 1000 km to suppress common- mode station errors. Under favorable turbulence conditions (r0 ∼ 20 cm), differential ranging precision at the tens-of-μm level is feasible. This “photon-rich” approach—combining high-power CW lasers, dual-wavelength ranging, narrowband filtering, and turbulence averaging—provides a scalable and precise alternative to pulsed-laser systems. Our findings establish high-power CW LLR as a transformative tool for advancing lunar geodesy, refining the lunar reference frame, and enabling next-generation tests of gravitational theories and low-frequency gravitational-wave detection.