Simple-structured, subfemtosecond-resolution optical-microwave phase detector.

Area of Science:

• Photonics
• Optical Engineering
• Precision Measurement

Background:

• Accurate measurement of timing differences between optical and microwave signals is crucial for advanced applications.
• Existing phase detection methods often require complex radio frequency (RF) electronics or magneto-optic components, limiting integration and simplicity.

Purpose of the Study:

• To demonstrate a simple, all-fiber photonic phase detector with subfemtosecond resolution.
• To achieve a low amplitude-to-phase conversion coefficient for robust phase measurements.
• To explore the potential for integrated photonic device implementation.

Main Methods:

• Utilized a Sagnac loop passively phase-biased by the intrinsic phase shift of a symmetric 3x3 fiber coupler.
• Employed an all-fiber design, eliminating the need for external RF electronics or magneto-optic modulators.
• Integrated the detector with a 250 MHz mode-locked Er-fiber laser and an 8 GHz microwave oscillator for synchronization testing.

Main Results:

• Achieved subfemtosecond resolution in measuring the phase difference between optical and microwave signals.
• Demonstrated a -60 dB amplitude-to-phase conversion coefficient.
• Attained a minimum residual phase noise floor of <-154 dBc/Hz (at 8 GHz carrier).
• Observed integrated root mean square (rms) timing jitter of 0.97 fs (1 Hz-1 MHz).
• Recorded long-term rms timing drift of 0.92 fs (over 5000 s) and frequency instability of 4x10^-19 (at 10,000 s averaging time).

Conclusions:

• The developed all-fiber photonic phase detector offers a simple and highly accurate method for optical-microwave synchronization.
• The passive biasing technique and all-fiber nature pave the way for compact, integrated photonic devices.
• The demonstrated performance metrics, including subfemtosecond resolution and low noise floor, are suitable for demanding timing applications.