Widely tunable optoelectronic oscillator based on a dual-frequency laser

Abstract

Summary form only given. Heterodyning of optical modes of a diode-pumped solid-state dual-frequency laser (DFL) is an efficient way to generate optically carried microwave signals [1], that can be used for the distribution of radar local oscillator, coherent or direct detection lidar or the implementation of stable and tunable optoelectronic oscillators. However, the free-running frequency stability of a DFL might be too low for the most demanding applications. We present here the stabilization of the beatnote of an Er, Yb:glass DFL at 1.53 μm with an optical frequency locked loop (OFLL) [2,3].The oscillator is schematized Fig. 1. Used as a voltage controlled oscillator (VCO), the DFL generates a widely tunable microwave signal from 0 to 14 GHz by heterodyning its two orthogonally polarized optical modes. The optical beatnote is detected on two different high-speed photodiodes, the first one directly at the laser output, the second one after an optical fiber coil inducing a delay τ. These two microwave signals are mixed together; we then obtain an error signal which is injected, through a baseband filter, to the DFL frequency control input. This results in the stabilization of the beatnote frequency on a multiple of 1/ τ. 100 m Phase Noise (dBc/Hz)With a 100 m optical fiber delay line, the beatnote phase noise is improved by 60dB leading to a level of -20 dBc/Hz, at 10 Hz from the carrier (Fig. 2), a fairly good result for such a short delay. The beatnote linewidth is then reduced from few hundreds of hertz to few hertz. Since this architecture does not require an RF filter, this oscillator is tunable from 2.5 to 5.5 GHz, actually limited by the RF components of our frequency discriminator. Such an oscillator could be used in a radar, communication or electronic warfare system requiring widely tunable microwave frequency references distributed via photonic links. Moreover, it could find applications in the laser radar area, either by using the wide tunability of the emitted signal to obtain a high range resolution in a direct detection system [4], or by providing a widely tunable local oscillator in a coherent detection lidar.