SYNCHRONIZER AND SYNCHRONIZING TRANSFER OVER AN OPTICAL LINK

FLC Business Search SYNCHRONIZER AND SYNCHRONIZING TRANSFER OVER AN OPTICAL LINK

SYNCHRONIZER AND SYNCHRONIZING TRANSFER OVER AN OPTICAL LINK

We disclose a “photon-efficient agile comb optical clock synchronizer” (PEACOCS) to support future ultra-precise time and/or frequency transfer over free-space links with ranges from 10’s to 1000’s of kilometers. Frequency-comb-based optical two-way time-frequency transfer can support frequency comparisons, time comparisons or the full synchronization between distant clocks at the sub-femtosecond level. The invention disclosed here allows for the synchronous operation of a tracking comb with a local oscillator comb to enable to retain sub-femtosecond timing while increasing the measurement signal-to-noise by ~ 5000X and thus the range by multiple orders of magnitude over existing methods.

PEACOCS contains at least two PEACOCS timing units which each consist of a pair of frequency combs, a timing discriminator and a signal processing unit. In its simplest configuration, one of these timing units is located at each end of a free-space optical link. Within each timing unit, one frequency comb serves as both the transmit and clock comb while the second comb serves as the “tracking” comb. The pulses from the transmit comb are sent over a free-space link to the timing unit located at the opposite side of the link. These transmitted comb pulses are received in the timing discriminator and optically combined with the tracking comb pulses. The rf output of the timing discriminator is directed to the signal processing unit, which returns the relative timing of the received comb pulses against the local tracking comb. These time offsets are both recorded and fed back to the local tracking comb so that its pulses remain overlapped in time to the received comb pulses. The tracking comb control allows for the signal processing unit to adjust the phase (time) of the comb pulses arbitrarily from femtoseconds to nanoseconds.

By performing this operation at both ends of an optical link, the time-of-flight fluctuations (from atmospheric turbulence or platform motion) can be suppressed and the time offset between the sites can be computed to better than a femtosecond. Alternately, the relative frequencies of the two sites can be computed at a fractional uncertainty of better than 10^-18.

As PEACOCS achieves this level of performance through the use of a tracking comb plus a specialized timing discriminator (described below), it provides a 5000X signal-to-noise boost over previously demonstrated methods without penalty in timing performance. The signal-to-noise boost is critical for long distance operation as the operational range is boosted by 70X for a given link power budget. This boost does come at the cost of increased complexity as now each timing unit requires two frequency combs instead of one, a more specialized detection unit, the timing discriminator, and active feedback for all modes of operation.
One of the significant challenges to PEACOCS is the detection of the arrival time of the received comb pulses from the remote site relative to the local tracking comb pulses. To this end, a PEACOCS timing unit can include one of several variants of the timing discriminator. First, the timing discriminator can consist of a “two-tap” interferometer constructed either out of free-space or fiber-optic components. The two-tap interferometer allows for the detection of the relative time offset of one pulse from another without the distortion of the measurement by amplitude variations. Second, the timing discriminator can consist of a “single-tap” interferometer plus a cw laser amplitude probe. In this case, simultaneous amplitude measurements of a cw laser allow for suppression of amplitude variation effects. Third, a “two-color” approach can be taken by splitting the frequency comb signals by optical frequency to again detect pulse offsets while suppressing the impact of amplitude fluctuations. Fourth, a dither can be introduced in the timing of the local oscillator comb and the average time offset over one cycle of this dither can be extracted via processing.

Another significant challenge for operation is the initial acquisition of the timing signals. The control hardware and algorithms for controlling the tracking combs supports the rapid switching between a swept acquisition mode and the steady-state operational mode to handle this.

Patent Abstract: 

We disclose a “photon-efficient agile comb optical clock synchronizer” (PEACOCS) to support future ultra-precise time and/or frequency transfer over free-space links with ranges from 10’s to 1000’s of kilometers. A PEACOCS timing units consists of a pair of frequency combs, a timing discriminator and a signal processing unit. Within each timing unit, one frequency comb serves as both the transmit and clock comb while the second comb serves as the “tracking” comb. The pulses from the transmit comb are sent over a free-space link to another timing unit located at the opposite side of the link. These transmitted comb pulses are received in the timing discriminator. The output of the timing discriminator is directed to the signal processing unit, which returns the relative timing of the received comb pulses against the local tracking comb. These time offsets are both recorded and fed back to the local tracking comb so that its pulses remain overlapped in time to the received comb pulses. The tracking comb control allows for the signal processing unit to adjust the phase (time) of the comb pulses arbitrarily from femtoseconds to nanoseconds.

Inventors: 

Sinclair, Laura Cathleen; Newbury, Nathan Reynolds; Swann, William Charles; Deschenes, Jean-Daniel

Internal Laboratory Ref #: 
20-033
Patent Status: 
Published Patent Application
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