Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation

TL;DR

This study investigates how the phase of microwave signals generated by InGaAs pin photodiodes from femtosecond laser pulses depends on optical pulse energy, revealing conditions for ultra-low phase noise microwave generation.

Contribution

It provides experimental measurements of amplitude-to-phase conversion in InGaAs photodiodes and identifies operating points that minimize phase noise in microwave signals.

Findings

Phase-to-energy coefficient alternates between positive and negative.

Vanishing points reduce amplitude to phase noise conversion.

Near vanishing points, low phase noise microwave signals are achievable.

Abstract

When a photo-diode is illuminated by a pulse train from a femtosecond laser, it generates microwaves components at the harmonics of the repetition rate within its bandwidth. The phase of these components (relative to the optical pulse train) is known to be dependent on the optical energy per pulse. We present an experimental study of this dependence in InGaAs pin photo-diodes illuminated with ultra-short pulses generated by an Erbium-doped fiber based femtosecond laser. The energy to phase dependence is measured over a large range of impinging pulse energies near and above saturation for two typical detectors, commonly used in optical frequency metrology with femtosecond laser based optical frequency combs. When scanning the optical pulse energy, the coefficient which relates phase variations to energy variations is found to alternate between positive and negative values, with many (for high harmonics of the repetition rate) vanishing points. By operating the system near one of these vanishing points, the typical amplitude noise level of commercial-core fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art ultra-low phase noise microwave signals, virtually immune to amplitude to phase conversion related noise.