Photonic Generation and Free-Space Distribution of Millimeter Waves for Portable Optical Clocks

TL;DR

This work addresses the challenge of disseminating sub-picosecond optical clock signals over open-air paths by introducing optically derived millimeter-wave carriers as a time–frequency link. It presents a practical architecture that generates stable millimeter waves from an optical clock output and demonstrates a phase-stabilized free-space link over 110 m with a fractional instability approaching the $10^{-14}$ level at $1$ s. The demonstrated residual millimeter-wave stability of $2\times 10^{-15}$ at $1$ s, combined with the free-space dissemination capability, provides a foundation for future time and frequency transfer among distributed portable optical clocks, with envisioned scaling to kilometer-scale links and integration on photonic chips for improved SWaP. The study identifies key limitations, notably out-of-loop fiber thermal drift, and outlines concrete steps to enhance robustness and reach for practical clock networks.

Abstract

Robust and portable optical clocks promise to bring sub-picosecond timing instability to smaller form factors, offering possible performance improvements and new scenarios for positioning and navigation, radar technologies, and experiments probing fundamental physics. However, there are currently limited methods suitable for broadly disseminating the sub-picosecond timing signals or performing frequency comparison of these clocks--particularly over open-air paths. Established microwave time transfer techniques only offer nanosecond level time synchronization, whereas optical techniques have challenging pointing requirements and lack the capability of all-weather operation. In this paper, we explore optically derived millimeter-wave carriers as a time-frequency link for full utilization of the next generation of portable optical clocks. We introduce an architecture that synthesizes 90 GHz millimeter waves with a one second residual instability of 2x10^-15, averaging into the 10^-17 range. In addition, we demonstrate a first-of-its-kind 110 m phase-stabilized free-space frequency comparison link over a millimeter-wave band with a one second instability in the 10^-14 region. Technical and systematic uncertainties are investigated and characterized, providing a foundation for future time and frequency transfer experiments among distributed portable optical clocks.

Figures (8)

• Figure 1: Time deviation of various time transfer techniques. Shown in the yellow region is a gap between flexible-pointing microwave techniques and optical free-space time transfer (OTT), the target region for millimeter wave based time and frequency transfer. In red, Integer Precise Point Positioning (IPPP), a GNSS based time transfer technique, is shown IPPP. In yellow, two-way satellite time transfer (TWSTT) is shown TWSTT. For context, a fiber based microwave over fiber (MOF) time distribution network is represented in black, but this approach is exclusively point-to-point for stationary sites ELSTAB. The purple trace shows the stability of OTT OTFT. Time deviation is used as these techniques are time transfer protocols with encoded timing information, not just frequency comparison techniques.
• Figure 2: An illustration of an envisioned millimeter wave time transfer network. Mobile and stationary platforms, each with an atomic clock, would be capable of one or two-way time transfer over kilometer scale links through turbulent weather.Gem
• Figure 3: a. Frequency domain illustration of stable millimeter wave generation. The clock laser is represented as frequency $\nu_0$, while the frequency comb is represented by the blue dashed lines. Locking beatnotes are represented by $f_{b0}$, $f_{b1}$, and $f_{b2}$. The ancillary fiber lasers are represented as frequencies $\nu_1$ and $\nu_2$. Millimeter wave frequencies are determined by the spacing of the fiber laser and the clock laser, denoted as $\nu_{m1}$ and $\nu_{m2}$. b. Experimental schematic for stable millimeter wave generation and characterization. A clock laser at $\nu_{0}$ is used to stabilize an optical frequency comb. The light carrying the comb and clock laser is then split, and heterodyned with two fiber lasers. Via a heterodyne beat, these lasers are phase locked to the nearest comb tooth. The fiber laser and clock lasers are then heterodyned on high bandwidth photodetectors, which can then be mixed to characterize stability. Optical signals in fiber are denoted in red, millimeter wave signals and components in light blue, and radio frequency and baseband signals and components are shown in black.
• Figure 4: Diagram of free-space millimeter wave phase stabilized link. Red and pink lines are optical paths, blue lines are millimeter wave paths, and black lines are low-frequency electrical paths. Optical tones $\nu_0$ and $\nu_1$ are sent to a fiber splitter, with one copy being sent to the transmit and one to the receive carts. The transmit cart, capable of pre-actuating out path-induced phase fluctuations, generates $\nu_{m1}$, which is broadcast and retroreflected to the receive cart. The receive cart uses this signal to close the feedback loop to stabilized the link. Another mixer on the receive cart is driven by either $\nu_{m1}$ or $\nu_{m2}$, leading to in-loop or out-of-loop analysis of the link performance.
• Figure 5: (a). MDEV of residual millimeter wave stability with the general transportable optical clock stability region seen in yellow. (b). Phase noise of millimeter wave beatnote. Transportable optical clock regions are defined by available and projected data sheets and papers from various companies and universities SeaClocksAussieClockVAVescentTiqkerQuantxTurnkey.