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Accueil > Équipes > OPTique et IMAgeries > Instrumentation et Méthodes > Time-frequency methods

Frequency shifting loops for microwave photonics

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1 Microwave photonics in frequency shifting loops (FSLs)

Frequency shifting loops are a simple photonic system based on a fiber loop, where the light is frequency-shifted at each roundtrip. Most of the physics of this system is described by two parameters : the roundtrip time of the light in the loop, and the value of the frequency shift per roundtrip. When seeded by a CW monochromatic laser, the FSL generates a comb of optical frequencies, with a quadratic spectral phase. The curvature of this spectral phase is proportional to the product of the two system’s parameters. When the CW laser is modulated by an input signal, the FSL creates replicas of the input signal, shifted both in time and in frequency. This property is at the origin of many applications of FSLs in microwave photonics. Since 2013, numerous unique properties of these systems have been demonstrated at LIPhy.

  • Under specific conditions, this system can be used as a source of low-jitter pulses with reconfigurable repetition rates (from 80 MHz up to 10 GHz). This behavior is related to a temporal Talbot effect, intrinsic to the system and its quadratic spectral phase. In collaboration with Thales Research and Technology in Palaiseau (Vincent Crozatier) and IES in Montpellier (Arnaud Garnache), we investigate the application of these pulsed sources for opto-electronic assisted sampling (PhD of Vincent Billault, CIFRE).
    See : GdC PRA (2013), GdC Opt Exp (2013), Billault PTL (2019), Billault JLT (2020).
    sketch of a FSL
    a : RF spectrum in different Talbot conditions. The rep. rates of the pulse trains are respectively close to 400 MHz (blue), 2.3 GHz (red), and 8 GHz (yellow). b : phase noise curves for the previous values of the rep. rate. c : comparison of the phase noise at low (resp. high) offset frequency, in blue (resp. red). d : hybrid Talbot laser combining a regenerative loop.

  • FSLs are a simple source of optical frequency combs, where the free spectral range (FSR) can be adjusted between 0, and 100 MHz. This frequency comb can be optimized, and used for multi-heterodyne interferometry, and dual-comb spectroscopy.
    See : Duran, OptExp (2018), Duran OptLett (2019), Kanagaraj OptExp (2020), Billault JOSAB (2020).
    sketch of a FSL
    a : Optical frequency combs in FSLs. The FSR is equal to 50 kHz (600 lines are clearly visible). Notice the influence of the seed power on the comb shape. b : application to the monitoring of FM radio stations.

  • FSLs have demonstrated astonishing capabilities for RF signal processing. We have implemented new functionalities, such as real-time Fourier, and fractional Fourier transformation. These results are based on an intrinsic frequency-to-time mapping, and reciprocal time-to-frequency mapping in FSLs.
    See : GdC Optica (2016), Schnébelin Optica (2017), Schnébelin OptLett (2020).
    sketch of a FSL
    a : Evidence of real-time Fourier transform in FSLs. The FSL is seeded by a phase modulated CW laser (i.e. carrier + 2 sidebands). The output time trace maps the input spectrum (frequency to time mapping). b : example of dual-comb spectroscopy in FSLs on an absorption line of HCN (1550 nm). c : evidence of fractional Fourier transform in FSLs. Top : experimental traces ; bottom : theoretical signals. d : evidence of time to frequency mapping in FSLs for short input signals : the output spectrum maps the input waveform.

  • • Another domain of applications of FSLs, is the generation of arbitrary signals. This can be achieved by using the frequency-to-time mapping in FSLs. In this case, one can provide arbitrary RF signals with bandwidth exceeding 20 GHz, strtaing from a low frequency input signal (80 MHz). Another possibility is the generation of arbitrary RF chirps, that have been used for lidar ranging by pulse compression.
    See : GdC NatComm (2018), Schnébelin NatComm (2019), Clement OptExp (2019), Clement OptExp (2020).
    sketch of a FSL
    a : Arbitrary signal generation by frequency to time mapping in FSLs. The output waveform reproduces Belledonne mountain (near Grenoble). b : broadband flat-top RF chirp generated in FSLs (time trace + Wigner-Ville transform). c : broadband waveform generated in FSLs with exponentially decaying envelope.