We review our experimental and simulation-modeling studies on optoelectronic oscillators (OEOs). The OEO can have
an intrinsic quality factor, Q that is orders of magnitude higher than that of the best electronic oscillators (i.e. Poseidon).
However, our experimental results show that the OEO's current phase noise level is still worse than that of the Poseidon.
This is caused by many noise sources in the OEO which reduce the "loaded-Q" in the loop system. In order to mitigate
these noise sources, we have systematically studied such phenomena as the laser RIN, Brillouin and Rayleigh scattering
in the fiber, vibration, etc. These noise sources are convoluted in both optical and electrical domains by many different
physical effects; hence, it is very difficult to experimentally separate them, and only the dominant phase noise is
observed in each offset-frequency. Therefore, we developed a computational model to simulate our experimental
injection-locked dual-OEO system. By validating the model with our experimental results from both individual
components and OEO loops, we can start to trace the individual phase noise sources. The goal is to use the validated
model to guide our experiments to identify the dominant phase noise in each spectral region, and mitigate these noise
sources so that the OEO can reach its full potential.
We report the first demonstration of a 10 GHz dual-fiber-loop Opto-Electronic Oscillator (OEO)
without RF-amplifiers. Using a recently developed highly efficient RF-Photonic link with RF-to-RF gain
facilitated by a high power laser, highly efficient optical modulator and high power phototectectors, we have
built an amplifier-less OEO that eliminates the phase noise produced by the electronic amplifier. The dual-loop
approach can provide additional gain and reduce unwanted multi-mode spurs. However, we have observed RF
phase noise produced by the high power laser include relative intensity noise (RIN) and noise related to the
laser's electronic control system. In addition, stimulated Brillouin scattering limits the fiber loop's length to
~2km at the 40mW laser power needed to provide the RF gain which limits the system's quality factor, Q. We
have investigated several different methods for solving these problems. One promising technique is the use of a
multi-longitudinal-mode laser to carry the RF signal, maintaining the total optical power but reducing the
optical power of each mode to eliminate the Brillouin scattering in a longer fiber thereby reducing the phase
noise of the RF signal produced by the OEO. This work shows that improvement in photonic components
increases the potential for more RF system applications such as an OEO's with higher performance and new
We have designed, assembled and tested a phase-array antenna using fiber Bragg gratings in the highly dispersive
transmission region as our tunable true-time delay (TTD) generators. The TTD generator is designed by cascading 29
identical fiber gratings and 1×2 fiber splitter pairs. Tapping from each fiber splitter allows us to steer our RF microwave
beam from a 30×4-element antenna by tuning the wavelength of a laser. The 10Ghz RF signal is superimposed upon a
laser beam by means of a LiNbO<sub>3</sub> modulator. However, with a conventional modulator, the optical frequency spectrum
of the modulated beam consists of two sidebands on opposite sides of the optical carrier; all three of which may
experience very different time delays due to dispersion. This may have detrimental effects on time-delay sensitive
processes such as antenna beamforming. Therefore, we studied the use of single-sideband versus double-sideband
modulators in the transmitters of photonic phased array antenna. We focus in particular on the effect of the different
spectral profiles of single and double sideband modulators on beamforming when using the fiber Bragg gratings as TTD
generators. With very high dispersion in our fiber Bragg gratings close to the band edge, the absolute propagation times
are different for each sideband and the optical carrier. Therefore double sideband modulated signals will generate two
sets of separate delays in the same microwave signal which causes beam deterioration and increased side-lobes. We
demonstrated this theoretically and verified experimentally by comparing the antenna patterns generated by single-side-band
and by double sideband modulators.
We present an application of fiber Bragg gratings as tunable optical delays in transmission for use as true-time-delay line in a RF-Photonic phased array antenna. Most delay line applications using fiber gratings require that they be used in reflection mode and they can provide only discrete variation of time delay. It also requires the use of bulky and expensive optical circulators. We have designed an optical true time delay array generator using fiber gratings in cascading transmission mode for such applications which significantly simplified the system and lowered the cost. A wavelength tunable laser is used as the light source. The laser light is modulated by an RF-microwave input signal, then enters into the optical true time delay array generator to provide a sequence of time delays Δt, 2Δt,...nΔt. The goal is to obtain large group delay Δt with low loss and with the capability of tuning Δt continuously by varying the wavelength of the laser. We combined an apodized grating profile, large index step and increased grating length to achieve our goal. We fabricated and tested the grating with about 100mm length which showed at least Δt=60 ps tunable time delay range. We have demonstrated the applicability of the transmission-mode fiber Bragg gratings in an optical true-time-delay type of phased array antenna.