High-bandwidth Coherence Cloning using Optical-Phase-Locking Feedforward

Abstract

Ultra-narrow-linewidth lasers with suppressed high-frequency phase noise are critical for quantum control and precision metrology. While optical phase locking (OPL) is the standard technique for cloning the coherence of such sources, its effectiveness is often limited at high frequencies by feedback latency. We present a robust feedforward architecture that overcomes this limitation by recycling and demodulating the existing master-slave beat signal to drive a single electro-optic modulator for near-instantaneous noise cancellation. This approach eliminates the extraneous sidebands and transmission losses typical of more complex modulators. Through active stabilization of the beat amplitude and demodulation phase, we demonstrate robust suppression exceeding 30 dB from 10 kHz to 10 MHz. This hardware-efficient framework is readily compatible with standard OPL setups, offering a scalable solution for high-fidelity coherent control.

Figures (5)

• Figure 1: Schematic of the OPL feedforward scheme. A standard optical phase-locked loop (gray area) locks the slave-master frequency offset, $\Delta\omega$. The feedforward stage (blue area) suppresses residual noise $\phi_r(t)$ outside the OPLL bandwidth by demodulating the photodiode (PD) beat with a second local oscillator (LO2) at frequency $\Delta \omega$, amplifying it with a constant-gain amplifier (P), and applying the resulting signal to a fiber electro-optic modulator (EOM). To ensure robustness, the beat amplitude is stabilized via a variable optical attenuator (VOA) (left green area), and the LO2 phase is adjusted to null the DC offset after the mixer (right green area). LP: low-pass filter.
• Figure 2: Feedforward efficacy versus noise frequency. (a) Reference-slave beat spectra with (FF) and without (no FF) feedforward, centered at 240 MHz with a 500 Hz resolution. Injected 2 MHz sinusoidal noise powers are indicated by black circles; their ratio determines the suppression shown in (b). Peaks at $\pm$240 kHz originate from the slave laser's intrinsic modulation, which is also effectively suppressed. Residual bumps above the -90 dBc floor result from the intensity noise of the reference laser. (b) Suppression as a function of injected noise frequency.
• Figure 3: Feedforward suppression performance for injected phase modulation at 1 MHz, recorded over a period of 24 hours.
• Figure 4: Performance of OPL feedforward at (a) $\Delta \omega=100$ MHz, (b) $\Delta \omega=200$ MHz, obtained with 500 Hz resolution.
• Figure 5: Detailed experimental setup. The green shaded area (a) encompasses the optical phase-locked loop and primary feedforward components. The beige shaded areas (b) include two feedback loops that enhance the robustness of the feedforward scheme. The blue shaded area (c) is used to characterize the feedforward effects. HWP, half-wave plate; PBS, polarizing beam splitter; FC, fiber coupler; VOA, variable optical attenuator; APD, avalanche photodiode; PID, proportional-integral-derivative controller; Splitter (Mini-Circuits ZFRSC-42-S+); RF PD, radio-frequency power detector (Mini-Circuits ZX47-40-S+); CPL, directional coupler (Mini-Circuits ZFDC-10-1+); Amp, RF amplifier (Mini-Circuits ZHL-72A+); SA, spectrum analyzer; LP, low-pass filter; P, home-built proportional amplifier.