Olukayode Okusaga

Spurious mode reduction in dual injection-locked optoelectronic oscillators

Optoelectronic oscillators (OEOs) are promising sources of low phase noise radio frequency (RF) signals. However, at X-band frequencies, the long optical fiber delay line required for a high oscillator Q also leads to spurious modes (spurs) spaced too narrowly to be filtered by RF filters. The dual injection-locked OEO (DIL-OEO) has been proposed as a solution to this problem. In this work, we describe in detail the construction of a DIL-OEO. We also present experimental data from our systematic study of injection-locking in DIL-OEOs. With this data, we optimize the DIL-OEO, achieving both low phase noise and low spurs. Finally, we present data demonstrating a 60 dB suppression of the nearest-neighbor spur without increasing the phase noise within 1 kHz of the 10 GHz central oscillating mode.

Comprehensive computational model of single- and dual-loop optoelectronic oscillators with experimental verification

We describe a comprehensive computational model for singleloop and dual-loop optoelectronic oscillators (OEOs). The model takes into account the dynamical effects and noise sources that are required to accurately model OEOs. By comparing the computational and experimental results in a single-loop OEO, we determined the amplitudes of the white noise and flicker noise sources. We found that the flicker noise source contains a strong component that linearly depends on the loop length. Therefore, the flicker noise limits the performance of long-cavity OEOs (≧5 km) at low frequencies (f<500 Hz). The model for a single-loop OEO was extended to model the dual-loop injection-locked OEO (DIL-OEO). The model gives the phase-noise, the spur level, and the locking range of each of the coupled loops in the OEO. An excellent agreement between theory and experiment is obtained for the DIL-OEO. Due to its generality and accuracy, the model is important for both designing OEOs and studying the physical effects that limit their performance. We demonstrate theoretically that it is possible to reduce the first spur in the DIL-OEO by more than 20 dB relative to its original performance by changing its parameters. This theoretical result has been experimentally verified.

Suppression of stimulated Brillouin scattering in optical fibers using a linearly chirped diode laser

The output of high power fiber amplifiers is typically limited by stimulated Brillouin scattering (SBS). An analysis of SBS with a chirped pump laser indicates that a chirp of 2.5 × 10(15) Hz/s could raise, by an order of magnitude, the SBS threshold of a 20-m fiber. A diode laser with a constant output power and a linear chirp of 5 × 10(15) Hz/s has been previously demonstrated. In a low-power proof-of-concept experiment, the threshold for SBS in a 6-km fiber is increased by a factor of 100 with a chirp of 5 × 10(14) Hz/s. A linear chirp will enable straightforward coherent combination of multiple fiber amplifiers, with electronic compensation of path length differences on the order of 0.2 m.

Rayleigh-Scattering-Induced RIN and Amplitude-to-Phase Conversion as a Source of Length-Dependent Phase Noise in OEOs

Optoelectronic oscillators (OEOs) are hybrid RF-photonic oscillators that promise to be environmentally robust frequency sources with very low phase noise. Recent experiments have shown that the excess flicker phase noise in these systems grows with the length of the optical fiber loop. In this paper, we detail a mechanism for this length-dependent flicker noise in which Rayleigh-scattering-induced amplitude noise in the optical fiber combines with amplitude-to-phase noise conversion in the nonlinear electronic components. We derive an analytic model of the loop noise that includes these effects and verify this model by comparing it to numerical calculations and experimental results.

Optical scattering induced noise in RF-photonic systems

Scattering mechanisms in optical fiber induce noise in the spectra of optical signals travelling through the fiber. This fiber-induced noise limits the quality factor of optical fiber resonators and degrades low-noise signals transmitted via optical fiber. In this work, we present preliminary data from our systematic study of optical scattering in fiber. We present noise spectra for Rayleigh and Brillouin scattering. We also demonstrate suppression of these scattering mechanisms via laser frequency modulation.

Experimental and simulation study of dual injection-locked OEOs

We investigate the physics of bidirectional injection-locking in optoelectronic oscillators (OEO). In particular, we identify the effects of injection strength on phase noise and spurious modes for a dual injection-locked OEO. Our experimental data is then used to design a numerical model of a dual injection-locked OEO. This model will be used in the future to optimize the multi-dimensional injection-locking parameters to achieve minimal phase noise and spurious mode levels.

Suppression of Rayleigh-scattering-induced noise in OEOs.

Optoelectronic oscillators (OEOs) are hybrid RF-photonic devices that promise to be environmentally robust high-frequency RF sources with very low phase noise. Previously, we showed that Rayleigh-scattering-induced noise in optical fibers coupled with amplitude-to-phase noise conversion in photodetectors and amplifiers leads to fiber-length-dependent noise in OEOs. In this work, we report on two methods for the suppression of this fiber-length-dependent noise: altering the amplitude-dependent phase delay of the OEO loops and suppressing the Rayleigh-scattering-induced noise in optical fibers. We report a 20 dB reduction in the flicker phase noise of a 6 km OEO via these suppression techniques.

Loop-length dependent sources of phase noise in optoelectronic oscillators

Optoelectronic oscillators are a promising source of spectrally pure, easily tunable microwave signals. These oscillators use a low-loss fiber optic delay line as a very high Q resonant cavity. However, length-dependent sources of phase noise prevent the full Q of the resonant cavity from being realized. Here we show evidence that this length-dependent phase noise is in part dependent upon the optical power and laser noise. This dependence is consistent with the conversion of laser noise to phase noise via the Kerr effect.

Superlinear growth of Rayleigh scattering-induced intensity noise in single-mode fibers

Rayleigh scattering generates intensity noise close to an optical carrier that propagates in a single-mode optical fiber. This noise degrades the performance of optoelectronic oscillators and RF-photonic links. When using a broad linewidth laser, we previously found that the intensity noise power scales linearly with optical power and fiber length, which is consistent with guided entropy mode Rayleigh scattering (GEMRS), a third order nonlinear scattering process, in the spontaneous limit. In this work, we show that this behavior changes significantly with the use of a narrow linewidth laser. Using a narrow linewidth laser, we measured the bandwidth of the intensity noise plateau to be 10 kHz. We found that the scattered noise power scales superlinearly with fiber length up to lengths of 10 km in the frequency range of 500 Hz to 10 kHz, while it scales linearly in the frequency range of 10 Hz to 100 Hz. These results suggest that the Rayleigh-scattering-induced intensity noise cannot be explained by third-order nonlinear scattering in the spontaneous limit, as previously hypothesized.

Non-ideal loop-length-dependence of phase noise in OEOs

We present an experimental study of the phase noise spectrums dependence on the loop length in OEOs. As the loop length increases, the spectrum deviates significantly from the ideal dependence.