Merging of Optoelectronic Techniques for Microwave Signal Generation A. Le Kernec, M. Varon and J.-C. Mollier Supaero-MOSE Toulouse, France arnaud.le_kernec@supaero.fr Abstract—This paper describes some recent evolutions about optical generation of microwave signals. Among these, multi- loop ring oscillator and harmonic generation allow both rise in frequency and wide tunability. Besides, using microresonators and optical sources like VCSEL (Vertical Cavity Surface Emitting Laser) induces better integration and lower power consumption. Taking advantage of these attractive features, a new architecture of photonic microwave source is proposed. I. INTRODUCTION Since many years, the need of high frequency microwave signals for telecommunication and datacom systems is growing. Many methods to generate them have been explored using, either totally electronic based architectures, or hybrid architectures, combining photonic and RF components. These last structures concentrate features inherent to photonic technology: possibility to generate very high frequency signals and the transportation of the signal to a remote location, taking advantage of the low losses of optical fibers. Several architectures have been proposed to optically generate microwave signals. The first one is the optical hererodyning using two frequency-offset lasers [1]-[2]. This is one of the most interesting techniques because of the high achievable frequencies and the wide tunability. Nevertheless, even if spectral purity and phase noise can be improved by using an optical phase-locked loop or an optical injection locking, the use of two lasers and the complexity of the feedback loop make this system power consuming and noisy. In the same way, a second technique is based on the beat- signal obtained from a bimode laser [3], whose interest is to correlate the frequency drifts of the two laser modes, but whose drawback is a weak accordability. The well-known optoelectronic oscillator using a long, fibered feedback loop provides a microwave signal presenting a very high spectral purity [4]. New configurations have appeared to rise in frequency. We review in this paper the dual-loop optoelectronic microwave oscillator and present a new technique of harmonic generation taking advantage of the good features of the ring oscillator. We present also two ways leading to a low-cost high integration of such systems, using a Vertical Cavity Surface Emitting Laser (VCSEL) and a microresonator. II. OPTICAL FIBER BASED MICROWAVE OSCILATOR A. Single loop microwave oscillator The optoelectronic microwave oscillator is composed of a loop containing a laser diode, a Mach Zehnder electrooptic modulator (MZM), an optical fiber, a photodetector, a microwave filter, and a microwave amplifier [4]. This is the architecture presented in Fig. 2 without taking into account the loop 2. The intensity modulated laser beam is injected into an optical fiber whose output is connected to a photodetector which generates the microwave signal. This signal is then filtered and amplified before being fed back to the RF modulation port of the MZM. Oscillations start thanks to noise sources described below. Only frequency components satisfying the phase and gain conditions are amplified in the loop and emerge from the noise floor. These modes appear at regularly spaced frequencies. The frequency space between two successive modes is the free spectral range (FSR), closely dependent of the length of the loop, equal to 1/[τ e +(n of .L/c)], where τ e is the delay due to electrical components, n of is the optical fiber index, L, its physical length and c is the speed of light in vacuum. The RF bandpass filter selects only one among these multiple oscillation modes. Using the Leeson's model [5], the phase-noise spectral density can be theoretically evaluated, taking into account the following noise sources contributing to the total intensity noise at the photodetector: • Relative Intensity Noise (RIN LAS ) of the laser source due to spontaneous emission. • Double Rayleigh scattering, modelized by a relative intensity noise RIN DRS and consisting in scattering twice of a fraction of the optical power by inhomogeneities in the optical fiber resulting in a weak delayed replica copropagating with the original optical wave. 172 1-4244-1168-8/07/$25.00 ©2007 IEEE.