Wide-range frequency drift compensation for tunable optoelectronic oscillator

Abstract

Optoelectronic oscillator (OEO) has attracted great interest during the last two decades. Due to its excellent performance, OEO can find wide applications in many fields. However, general OEO cavities are sensitive to thermal fluctuation; this character could greatly deteriorate the long-term frequency stability of OEO. To solve this problem, thermal stabilization, injection-locking and phase-locked loop (PLL) have been widely employed. Nevertheless, increased bulk, deteriorated phase-noise performance, small operating temperature range, or limited bandwidth brings new problems for tunable OEOs. In this paper, two wide-range broadband frequency-drift compensating methods have been proposed to solve those problems. In the first method, we extracted the time delay fluctuation of OEO's optical loop via a low frequency calibration signal, and offset the fiber's random delay fluctuation dynamically with a tunable laser. In the second method, we demonstrated a novel broadband and wide-range feedback tuning scheme for PLL stabilization of tunable OEOs based on a dual parallel Mach-Zehnder modulator and optical bandpass filter. The wide range and broadband performance of the proposed methods were demonstrated theoretically and experimentally, which ensured OEO's wide operating temperature range and tunability for further applications. As a result, stabilization for OEO was achieved at different oscillating frequencies and the long-term stability was greatly improved without any thermal control. The measured overlapping Allan deviation of stabilized OEO was two orders of magnitude better than the free-running OEO in the first method, and four orders of magnitude better than that in the second one. Meanwhile, phase noise performance remained the same at high-offset frequencies.