Wide Band & high resolution optical coherent processing of RF signals in Er:YSO using a frequency agile laser

Abstract

Various microwave photonic processors using rare-earth-ion-doped crystals are under development, driven by the multi-GHz bandwidth and sub-MHz resolution potential of these crystals. Exploiting the available wide spectral band has long been a challenge, and only recently have some applications succeeded to. These include spectral analysis, range-Doppler processing. In these processors, the wide band advantage requires laser chirps covering the desired bandwidth in a few milliseconds with a precision better than the resolution aimed at. This is challenging. However in these applications one need not the chirp to be phase coherent all along its duration. Indeed, no phase relation need be conserved between the different frequency classes. Some coherent processing schemes require such phase relation. One of these is the so called photon echo chirp transform process [1], which can be used for instantaneous spectral analysis, compressive receivers, and also for arbitrary waveform generation. Up to now no convincing wide band demonstration could be performed of this coherent spectral process, because producing phase coherent, GHz-wide optical chirps of a few microseconds duration only is very challenging. We present here the first wide band and high resolution demonstration of the chirp transform algorithm. Because it is performed with a frequency agile laser source, this demonstration truly opens the way to the wealth of wide band spectral coherent processes that rare-earth ion doped crystals can handle.Experiments are performed with the 1536 nm transition of an Er3+:Y2SiO5 crystal, whose inhomogeneous width and dephasing time are 2 GHz and 150 pts respectively in our experimental conditions. Given these spectral parameters, one needs a laser whose frequency can be swept on several GHz in -20 Its, with a precision better than 50 kHz. We built an extended cavity diode laser with an intra-cavity electro-optic (EO) crystal, phase locked on an unbalanced MachZehnder interferometer [2]. Using this source, we demonstrated the chirp transform process with a bandwidth in excess of 1 GHz together with more than 20,000 independent frequency channels. (a) 200 kHz (b) 1.0 05 0.4 0.71 200k (b) i7oo0 0.6-0.8-~~~~~~~~~~~~. (U Frequency (Hz) 0.6 00 50kkHz 0.4-~~~-~4~~~~~0 .4~~~~~~~. 0.0 H0.0-0 0.25 0.5 0.75 1 1.25 1.5 -42 0 2 4 6 8 10 Frequency (GHz) Time (ns) Fig. 1 (a) 1.5-GHz wide RF spectrum. The line is the spectrum given by our photon echo processor. The dots are the peaks height evaluated from the FFT of the RF signal. The inset shows the peak of the dashed part. The peak width is 67 kHz, Fourier-limited by the 15-his analysis duration. (b) Fourier transform of a 20-his long gate, sine modulated at 100 kHz.As can be seen on Fig 1(a) a 1.5-GHz wide spectrum can be analyzed in 15 Its, leading to a resolution of 67 kHz.The time-bandwidth product of 24,000 is, to our knowledge, the highest ever demonstrated for a photon-echo processor. The dynamic range is 25 dB in single shot regime, and can be raised to 32 dB when the spectrum is...