Real-time measurement of complex fast signals by bandwidth compression in frequency shifting loops.

We report coherent time-to-frequency mapping in frequency shifting loops (FSLs). We show that when seeded by a temporal signal shorter than the inverse of the frequency shift per roundtrip, the optical spectrum at the FSL output consists of a periodic replica of the input waveform, whose temporal amplitude and phase profiles are mapped into the frequency domain. We provide an experimental demonstration of this phenomenon and show how this simple setup enables real-time measurement of fast non-repetitive input RF signals with a detection chain two orders of magnitude slower than the input signal.

Programmable broadband optical field spectral shaping with megahertz resolution using a simple frequency shifting loop.

Controlling the temporal and spectral properties of light is crucial for many applications. Current state-of-the-art techniques for shaping the time- and/or frequency-domain field of an optical waveform are based on amplitude and phase linear spectral filtering of a broadband laser pulse, e.g., using a programmable pulse shaper. A well-known fundamental constraint of these techniques is that they can be hardly scaled to offer a frequency resolution better than a few GHz. Here, we report an approach for user-defined optical field spectral shaping using a simple scheme based on a frequency shifting optical loop. The proposed scheme uses a single monochromatic (CW) laser, standard fiber-optics components and low-frequency electronics. This technique enables efficient synthesis of hundreds of optical spectral components, controlled both in phase and in amplitude, with a reconfigurable spectral resolution from a few MHz to several tens of MHz. The technique is applied to direct generation of arbitrary radio-frequency waveforms with time durations exceeding 100 ns and a detection-limited frequency bandwidth above 25 GHz.

Optimization of acousto-optic optical frequency combs.

Acousto-optic optical frequency combs can easily produce several hundreds of mutually coherent lines from a single laser, by successive frequency shifts in a loop containing an acousto-optic frequency shifter. They combine many advantages for multi-heterodyne interferometry and dual-comb spectroscopy. In this paper, we propose a model for an intuitive understanding of the performance of acousto-optic optical frequency combs in the steady state. Though relatively simple, the model qualitatively predicts the effect of various experimental parameters on the spectral characteristics of the comb and highlights the primordial role played by the saturation of the gain medium in the loop. The results are validated experimentally, offering a new insight in the performance and optimization of acousto-optic frequency combs.

Laser ranging using coherent pulse compression with frequency shifting loops.

We report on the use of the acousto-optic frequency combs generated by frequency shifting loops as compact and versatile optical waveforms generators for pulse compression systems in the optical coherent domain. The high degree of tunability and mutual coherence of these sources permits an efficient use of the available detection bandwidth, and represent simple alternatives to broadband lasers that do not require fast electronics. The full, complex optical field is retrieved using heterodyne measurements in bandwidths as high as 20 GHz. Compression ratios up to 150 at 80-MHz repetition rate, with autocorrelation peak-to-sidelobe ratios in excess of 28 dB, are demonstrated. In a proof-of-concept ranging experiment, we obtain resolutions of 4 mm in free space at meter scales, limited by detection bandwidth. Systems based on frequency shifting loops thus enable compact implementations of the pulse compression concept in the optical coherent domain, for its use in general optical metrology systems.

Measurements of the third- and fifth-order optical nonlinearities of water at 532 nm and 1064 nm using the D4σ method: erratum.

This erratum corrects an error in Fig. 4 of Opt. Lett. 39, 5046 (2014).

Reconfigurable photonic generation of broadband chirped waveforms using a single CW laser and low-frequency electronics.

Broadband radio-frequency chirped waveforms (RFCWs) with dynamically tunable parameters are of fundamental interest to many practical applications. Recently, photonic-assisted solutions have been demonstrated to overcome the bandwidth and flexibility constraints of electronic RFCW generation techniques. However, state-of-the-art photonic techniques involve broadband mode-locked lasers, complex dual laser systems, or fast electronics, increasing significantly the complexity and cost of the resulting platforms. Here we demonstrate a novel concept for photonic generation of broadband RFCWs using a simple architecture, involving a single CW laser, a recirculating frequency-shifting loop, and standard low-frequency electronics. All the chirp waveform parameters, namely sign and value of the chirp rate, central frequency and bandwidth, duration and repetition rate, are easily reconfigurable. We report the generation of mutually coherent RF chirps, with bandwidth above 28 GHz, and time-bandwidth product exceeding 1000, limited by the available detection bandwidth. The capabilities of this simple platform fulfill the stringent requirements for real-world applications.

Spectral interpretation of Talbot self-healing effect and application to optical arbitrary waveform generation.

The Talbot effect has the intrinsic capability of mitigating the effects of grating imperfections. Here we investigate, in the frequency domain, the influence of an imperfect grating on the Talbot images. In particular, we show that for specific propagation distances, the spectra of the individual Talbot images are directly related to the envelope of the grating transmission function, mapped in the spectral domain. This novel space-to-frequency mapping property enables us to describe the capability of image restoration (or self-healing) of the Talbot effect, in terms of spectral filtering. The transposition of this effect from the spatial to the temporal domain offers promising possibilities for the spectral shaping of optical waveforms.

Measurements of the third- and fifth-order optical nonlinearities of water at 532 and 1064 nm using the D4σ method.

The nonlinear response of liquid water was investigated at 1064 and 532 nm using a Nd:YAG laser delivering pulses of 17 ps and its second harmonic. The experiments were performed using the D4σ method combined with the Z-scan technique. Nonlinear refractive indices of third- and fifth-order were determined, as well as the three-photon absorption coefficient, for both wavelengths. A good agreement was found between theory and experiment.