Two-frequency Solid-state Laser Research Articles

Terahertz Optoelectronic Down-Conversion and Phase-Locking Through Four-Wave Mixing

Optoelectronic down-conversion of a terahertz optical beatnote to a RF intermediate frequency is performed with a standard Mach-Zehnder modulator followed by a zero dispersion-slope highly nonlinear fiber. The two interleaved optical combs obtained by four-wave mixing are shown to contain more than 75 harmonics enabling to conveniently recover the phase noise of a beatnote at 770 GHz at ~500 MHz. This simple four-wave mixing assisted down-conversion architecture is implemented to a two-frequency solid-state laser in order to directly phase-lock its frequency difference. This is illustrated on a beatnote at 168 GHz directly phase locked to the local oscillator at 10 MHz.

Phase and intensity dynamics of a two-frequency laser submitted to resonant frequency-shifted feedback

We propose an experimental and theoretical study of the dynamical regimes observed when the two modes of a two-frequency solid-state laser are coupled by frequency-shifted optical feedback. The detuning associated to the feedback is close to the frequency of the laser relaxation oscillations. Special attention is devoted to the dynamics of the phase of the beat note between the modes relative to the phase of an external radio-frequency reference. In particular, we analyze in detail the transition from phase locking to the external reference to phase drift. This transition occurs through a window of frequency locking without phase locking [Th\'evenin, Romanelli, Vallet, Brunel, and Erneux, Phys. Rev. Lett. 107, 104101 (2011)]. Furthermore, a large variety of dynamical behaviors, such as weak intensity modulation, self-pulsing, and chaos, are experimentally found. These results are successfully reproduced by a theoretical model based on coupled rate equations. The extension of the model to the description of other active media such as semiconductor lasers is briefly outlined.

耦合腔结构可调谐双频固体激光器的研究

耦合腔结构可调谐双频固体激光器的研究

Beat-note locking in dual-polarization lasers submitted to frequency-shifted optical feedback

We derive a delay-differential equation model that describes continuous-wave (cw) or passively Q-switched (PQS) two-frequency solid-state lasers submitted to frequency-shifted feedback (FSF). The study focuses on the locking of the beat note between the two free-running laser frequencies to a reference external frequency. The locking domain is obtained analytically in the cw regime. The PQS regime is treated by adding a saturable absorber population in the model equations. In this case, numerical simulations permit us to evaluate a locking range that is smaller than in the cw case. We find good agreement between the theoretical predictions and experiments carried out with a cw diode-pumped dual-polarization Nd:YAG laser as well as with previously published experimental results obtained with cw Er:Yb:glass [ Opt. Lett. 32, 1099 (2007)] and PQS Nd:YAG [ Opt. Lett. 33, 2524 (2008)] lasers. Applications of the FSF locking technique include the lidar-radar technique, for which a highly coherent beat note is required.

The study on diode-pumped two-frequency solid-state laser with tunable frequency difference

The laser diode pumped two-frequency solid-state laser with tunable frequencydifference is investigated, the medium is a 1at% doped 2-mm thick Nd:YAG chip. The condition of stable two-frequency oscillation operation is analyzed, the polarization is calculated by Jones matrix in different locations at the resonator. It is proved that the two-frequency electric field vectors are orthogonally polarized everywhere, the frequency difference tuning can be realized by changing the angle between the fast axes of two quarter-wave plates. In the experiment, single-frequency oscillation was obtained with the help of an etalon, then longitudinal mode frequency splitting resulted in the two-frequency oscillating operation by inserting two quarter-wave plates. The obtained maximum frequency difference was 1.3GHz, the minimal was 50MHz because of the strong mode competition. Due to the loss anisotropy of the etalon, the output power is not very stable when tuning the frequency difference, 85mW two-frequency output power was obtained with 100MHz beat note.

High-Spectral Purity RF Beat Note Generated by a Two-Frequency Solid-State Laser in a Dual Thermooptic and Electrooptic Phase-Locked Loop

Ultralow noise stabilization of a two-frequency solid-state laser is experimentally demonstrated using a specially designed dual phase-locked loop. Thanks to an intracavity birefringent crystal providing both thermooptic and electrooptic frequency modulation properties, the laser delivers a beat frequency that is controlled by both pump power and voltage from 0 to 1000 MHz. When phase-locked against a radio-frequency synthesizer, the beat note has a 25-mHz full-width at half-maximum. The measured phase noise is -110 dBc/Hz at 10-kHz offset, and lower than -130 dBc/Hz at 1-MHz offset and above. Improvements and applications to local oscillators in microwave-photonic systems are discussed.

Coherence of pulsed microwave signals carried by two-frequency solid-state lasers

This paper studies the coherence of the radio-frequency (RF) beat note between the two frequencies emitted by a two-eigenstate solid-state laser operating in pulsed regime. Depending on how the pulsed operation regime is obtained-passive Q switching using a saturable absorber or resonant amplitude modulation using pump-power modulation-we show theoretically and experimentally that the RF beat note loses or maintains its coherence from pulse to pulse. Namely, RF coherence is shown to be lost when the laser intensity vanishes between pulses, while it can be maintained if a slight amount of intensity remains inside the cavity between pulses. We give experimental demonstrations of these results for both Nd:YAG and Er-Yb:glass two-frequency pulsed lasers, in connection with applications to lidar-radar systems.

Two-frequency solid-state lasers with intracavity second-harmonic generation

A method for stabilizing two-frequency lasing in a solid-state laser with isotropic cavities is proposed. The method is based on intracavity second-harmonic generation. The method is analyzed. Conditions for the stability of two-frequency lasing during intracavity second-harmonic generation are found for nonlinear crystals with type II phase matching.