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In this paper, the proof-of-concept of an optical fiber sensing technique based on a frequency shifting loop (FSL) is proposed. The system combines both a refreshing rate of the sensing measurement in the MHz range and a centimetric spatial resolution. The FSL is used as a versatile source of broadband optical chirped waveforms, with controllable chirp rate. Digital pulse compression is applied to optimize the system's signal-to-noise-ratio. The sensor system is used to monitor the phase perturbation applied to a localized transducer (an electro-optic phase modulator) placed in an optical fiber. A preliminary and promising sensitivity

We report on a detailed analysis of the effect of CW injection in an actively mode-locked laser at 780 nm, on the noise performances of the laser. The optical properties of the emitted pulse train, and the phase and amplitude noise of the RF pulse train generated after photodetection have been characterized with respect to the laser pumping rate, and to the injected power. We show that when injected, the laser exhibits a dramatic improvement of both RF phase noise and power spectrum, but at the expense of a degraded optical relative intensity noise.

High repetition rate optical pulse trains are essential for a wide range of applications, including photonic-assisted sampling. Among the different solutions proposed so far, Talbot lasers, based on the generation of an optical frequency comb in a frequency shifting loop, are a simple yet efficient source of short transform-limited pulses. By controlling the comb spectral phase, the repetition rate is reconfigurable. It can be set to a (possibly large) multiple of the comb spacing and reach the GHz range, while only requiring a low bandwidth synthesizer (typ. 100 MHz). The noise characteristics of this pulse trains is a key factor for its application. Therefore, we investigate the phase noise properties of Talbot lasers. In particular, we prove theoretically and demonstrate experimentally that the phase noise at high offset frequency from the carrier, does not depend on the multiplication factor $q$ . We also demonstrate that the phase noise at low offset frequencies, which increases as $20 \log q$ , can be strongly reduced by a simple locking technique. Finally, we show that in addition to their reconfigurability, Talbot lasers can provide timing jitters at 8.2 GHz as low as 350 fs (integrated from $10^{5}$ Hz to the Nyquist frequency), making them suitable candidates for time metrology applications.

International audience; We demonstrate a new and simple dynamic laser ranging platform based on analog all-optical coherent pulse compression of modulated optical waveforms. The technique employs a bidirectional acousto-optic frequency shifting loop, which provides a dual-comb photonic signal with an optical bandwidth in the microwave range. This architecture simply involves a CW laser, standard telecom components and low frequency electronics, both for the dual-comb generation and for the detection. As a laser ranging system, it offers a range resolution of a few millimeters, set by a dual-comb spectral bandwidth of 24 GHz, and a precision of 20 µm for an integration time of 20 ms. The system is also shown to provide dynamic measurements at scanning rates in the acoustic range, including phase-sensitive measurements and Doppler shift velocimetry. In addition, we show that the application of perfect correlation phase sequences to the transmitted waveforms allows the ambiguity range to be extended by a factor of 10 up to ∼20 m. The system generates quasi-continuous waveforms with low peak power, which makes it possible to envision long-range telemetry or reflectometry requiring highly amplified signals.

In radar technology, the term pulse compression refers to a set of techniques that allows achieving the resolution of short and high peak-power pulses by transmitting long signals with relatively low peak powers [1] . This result is possible thanks to the use of modulated or coded waveforms with an enlarged time-bandwidth product, which can be easily amplified without the onset of nonlinearities. After these waveforms are sent to a remote target, the cross-correlation (CC) of the detected back-reflections with a reference signal (in a process called matched filtering) produces narrow pulses, enabling a range resolution set by the bandwidth of the transmitted signal. So far, the concept of pulse compression, when extended to the the optical counterpart of radar (lidar), has been mostly limited to incoherent optical waveforms coded in intensity. In this paper, we present a scheme for laser ranging by all-optical coherent pulse compression. It consists in a bidirectional frequency shifting loop (FSL), composed of a couple of acousto-optic frequency shifters (AOFS1 and AOFS2), an optical amplifier (EDFA) and a tunable bandpass filter (TBPF), see Fig. 1a [2] . The system is configured so that the recirculation of the light inside the loop produces two optical frequency combs (OFCs) with quadratic spectral phases, and whose frequency spacings differ by a small quantity δf. By a precise adjustment, the FSL generates two trains of optical chirps with similar chirp rate but slightly different period. The interference of these quasi-continuous waveforms on a photodetector (PDcc), followed by a low-pass filter, produces the matched filtering in a purely analog manner. Simultaneously, the dual-comb interference enables to reduce by orders of magnitude the detection electronics bandwidth.

Atmospheric turbulences can generate scintillation or beam wandering phenomena that impairs free space optical (FSO) communication. In this paper, we propose and demonstrate a proof-of-concept FSO communication receiver based on a spatial demultiplexer and a photonic integrated circuit coherent combiner. The system collects the light from several Hermite Gauss spatial modes and coherently combine on chip the energy from the different modes into a single output. The FSO receiver is characterized with a wavefront emulator bench that generates arbitrary phase and intensity patterns. The multimode receiver presents a strong resilience to wavefront distortions, compared to a monomode FSO receiver. The system is then used to detect a modulation of the optical beam through a random wavefront profile., Comment: 8 pages, 7 figures

An optical fiber sensing system with a frequency shifting loop monitor phase perturbations applied to a transducer in an optical fiber. A sensitivity below 4 µrad/Hz^(1/2) in a 20 kHz – 2 MHz bandwidth is obtained.

We perform laser ranging using a dual-comb system that generates trains of optical chirps with slightly different periods. It allows analog pulse compression with low-frequency electronics and the possibility of expanding the ambiguity range.

We propose and demonstrate a novel approach of mode locked laser to generate pulse trains at high repetition rate, with low phase noise and broadband tunability. The architecture takes advantage of regenerative mode locking to enhance the frequency selectivity by more than 25 dB and the noise performances of rational mode locking. The output pulse train phase noise combines ideal frequency multiplication of the RF synthesizer at low offset frequency (up to 10 kHz) and low residual phase noise of regenerative lasers at higher offset frequencies.

Recently, methods based on all-optical pulse rate multiplication have emerged to overcome the limitation of standard RF synthesiser for the generation of high frequencies. Among them, Frequency Shifting Loops (FSLs) injected by a CW laser, so called Talbot lasers, can generate pulse trains with a repetition rate tunable over several order of magnitudes (up to tens of GHz) from a low frequency synthesiser [1]. Until now, Talbot lasers have been theoretically described in the frame of a passive cavity in the steady state. Here, we present a simple model based on rate equations dedicated to the dynamics of Talbot lasers. This model allows to successfully explain experimental results with both solid-state and semi-conductor gain media, without parameter adjustment.

Actively and harmonically mode locked lasers (AHMLL) have been extensively studied as an optical clock source for photonic-assisted analog-to-digital conversion of broadband RF signals. Such lasers can generate high repetition rate pulse train with ultralow timing jitter [1]. The optical injection of AHMLL has been shown to be an interesting way to reduce the supermode noise, which is an important source of residual phase noise at high offset frequency [2]. Although efforts have been put to stabilize the external seed laser on a longitudinal mode of the AHMLL [3], no study on the impact of seed laser parameters have been reported. We present here our first results on the parametric study of a CW-injected AHMLL to understand better the evolution of the laser performance under different power and optical gain conditions.

Adaptive holographic interferometry is a promising method for high-sensitivity phase modulation measurements in the presence of slow perturbations from the environment. This technique is based on the use of a nonlinear recombining medium. Here, we report the use of an adaptive holographic interferometer based on an optically addressed spatial light modulator. Owing to the physical mechanisms involved, the interferometer adapts to slow phase variations within a range of 5–10 Hz. Moreover, owing to the basic principle of holography, this technique can be used with complex wave fronts such as the speckled field reflected by a highly scattering surface or the optical field at the output of a multimode optical fiber. We demonstrate both theoretically and experimentally, that using a multimode optical fiber as sensing element, rather than a single mode fiber, allows improving the interferometer phase sensitivity. Finally, we present a phase-OTDR optical fiber sensor using the adaptive holographic interferometer.

The understanding of frequency-shifting loops’ (FSLs) transient response is fundamental for their implementation in microwave photonic systems. We developed a numerical model, enabling us to describe the specific dynamics of the frequency comb generated in a FSL seeded by a CW laser. The model, based on laser rate equations, predicts the temporal evolution of the power of the individual comb lines during on/off cycles of the injection power, for FSLs operating below the lasing threshold. To prove the validity of the model, we performed experimental measurements of the power of the individual comb lines with time. Numerical simulations based on the model, for different gain media and pumping rates, are in excellent agreement with the experimental results. This paper provides guidance for the concrete implementation of FSLs for microwave photonic applications.

We propose and demonstrate an improved Talbot laser including a regenerative RF feedback loop. This architecture greatly enhances both amplitude and noise performances of the pulsed laser, while preserving the broadband tunability of its repetition rate. The phase noise power spectral density of the associated RF signal is found to be independent of the repetition rate. For a 4.9 GHz repetition rate, the timing jitter is as low as 54 fs (integrated between 10 kHz and 100 MHz offset frequency). This architecture paves the way towards tunable high performances coupled opto-electronic microwave oscillators.

We report on the generation of GHz pulse trains with a regenerative frequency shifting cavity at 780 nm using a semiconductor optical amplifier. This configuration improves the sidemode rejection, and delivers pulse train with mW power.