Jérémie Thévenin

Dual-polarization mode-locked Nd:YAG laser

A mode-locked solid-state laser containing a birefringent element is shown to emit synchronously two frequency combs associated to the two polarization eigenstates of the cavity. An analytical model predicts the polarization evolution of the pulse train, which is determined by the adjustable intracavity birefringence. Experiments realized with a Nd:YAG laser passively mode locked by a semiconductor saturable absorber mirror are in perfect agreement with the model. Locking between the two combs arises for particular values of their frequency difference, e.g., half the repetition rate, and the pulse train polarization sequence is then governed by the relative overall phase offset of the two combs.

Lidar–radar velocimetry using a pulse-to-pulse coherent rf-modulated Q-switched laser

An rf-modulated pulse train from a passively Q-switched Nd:YAG laser has been generated using an extra-cavity acousto-optic modulator. The rf modulation reproduces the spectral quality of the local oscillator. It leads to a high pulse-to-pulse phase coherence, i.e., phase memory, over thousands of pulses. The potentialities of this transmitter for lidar-radar are demonstrated by performing Doppler velocimetry on indoor moving targets. The experimental results are in good agreement with a model based on elementary signal processing theory. In particular, we show experimentally and theoretically that lidar-radar is a promising technique that allows discrimination between translation and rotation movements. Being independent of the laser internal dynamics, this scheme can be applied to any Q-switched laser.

Dual-polarization frequency comb from a diode-pumped solid-state laser

A mode-locked laser containing two quarter-wave plates emits synchronously two combs associated to the polarization eigenstates of the cavity. Experiments are in agreement with a modal analysis predicting the polarization sequences of the pulse train.

RF-modulated optical pulses generated by non-resonant frequency-shifted feedback for Lidar-Radar velocimetry

Lidar-Radar detection is based on the use of an optically-carried radiofrequency (RF) signal in order to benefit from both the directivity of the optical beam (Lidar), and the accuracy of RF signal processing (Radar) [1]. For many applications, the use of modulated optical pulses becomes mandatory since the detection is quadratic and the optical power of cw lasers is relatively low. Meanwhile, the pulse-to-pulse coherence has to be preserved to efficiently measure the target velocity from the Doppler shift on the RF [2]. We report on the use of an external feedback cavity containing an acousto-optic modulator to generate a RF-modulated pulse train from any Q-switched laser without any electronics feedback loop. A demonstration of a Doppler velocimetry measurement is performed.

Extended synchronization resulting from resonant phase and intensity dynamics in a dual-polarization laser

We study experimentally and numerically the synchronization dynamics of two laser modes, coupled by a coherent optical feedback injection. For a given coupling strength, we observe three synchronization regimes, depending on the initial detuning between the modes. If this detuning is sufficiently weak, the phase of the beat between the two modes remains constant. In contrast, for very large detuning, this relative phase drifts. Besides these two familiar cases, we identify an intermediate situation, in which the relative phase oscillates while remaining bounded. The frequency locking is preserved, even in the absence of phase locking.

New forms of synchronization dynamics of a dual-polarization laser with frequency-shifted feedback

Dual-frequency lasers offer beat notes from DC to the terahertz range [1], leading to various applications such as remote sensing, lidars, and Doppler velocimetry. In order to stabilize the beat note in a dual-frequency laser, a new scheme consisting in feeding back one polarization mode into the other was recently developed [2]. Here, we investigate experimentally, numerically, and analytically the synchronization properties between the two polarization modes when the locking range is comparable to the relaxation oscillation frequency of the laser. An important parameter is the coupling coefficient β that measures the cross-saturation of the two polarization modes in the gain medium. If β → 0, the two-mode laser system reduces to a single mode laser subject to an injected signal [3]. On the other hand, if β → 0 (β = 0.8 in our experiments), we observe more complex forms of synchronizations that strongly depend on the intensities of the individual modes.