Self-pulsing Behavior Research Articles

Red Sm:KGd(WO4)2 laser at 649 nm.

We report on the first laser operation of a Sm3+-doped monoclinic KGd(WO4)2 double tungstate crystal in the red spectral range. Pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) at 479.1 nm, the 0.8 at.% Sm:KGd(WO4)2 laser generated an output power up to 17.6 mW at 649.1 nm (the 4G5/2 → 6H9/2 transition) with a slope efficiency of 16.9%, a laser threshold down to 29 mW and a linear polarization. The laser exhibited a self-pulsing behavior, delivering µs-long pulses with a repetition rate of a few kHz. The polarized spectroscopic properties of Sm3+ ions were determined as well.

Thulium-doped all-fiber laser oscillator with more than 100 W of output power at 1940 nm and study of self-pulsing behaviour

Thulium-doped all-fiber laser oscillator with more than 100 W of output power at 1940 nm and study of self-pulsing behaviour

Optical instabilities in Fabry-Perot resonators

We start from the Maxwell-Bloch equations that govern the dynamics of Fabry-Perot lasers and, considering a doubled cavity, generalize our traveling wave formalism to the case in which the two-level medium does not fill the cavity but occupies only a portion of it. We linearize these equations and focus on the adiabatic elimination of both atomic fluctuations. Next, we restrict our attention to free running lasers under resonant conditions and analyze amplitude and phase instabilities. The examination of the unstable domain leads to the conclusion that the multimode Fabry-Perot instability arises near threshold only when the ratio of the longitudinal to the transverse atomic relaxation rate is substantially smaller than unity. This result agrees with our previous study in the limit of adiabatic elimination of the atomic polarization fluctuations only. We describe the self-pulsing behavior that arises from the multimode instability, and exhibits hysteretic behavior when the pump parameter is swept forward and backward. Finally, we investigate the single-mode instability and show that in the Fabry-Perot case there is no longer the correspondence between single-mode and multimode instabilities that is well known in the case of ring lasers. We confirm that in the case of resonant Fabry-Perot lasers the single-mode amplitude instability arises very far from threshold.

An investigation into self-pulsing behavior in an Er-doped ring laser

We observe a strong self-pulsing phenomenon in an all-fiber Er-doped ring laser. With an increase in the pump power, the generated pulse gradually evolves from a sine-wave pulse to a typical mode-locking pulse train. The self-mode-locked laser pulse obtained has a pulse-repetition rate in the megahertz region and a maximum average output power of around 1 mW. The self-pulsing evolving dynamics can be analyzed using coupled-wave rate equations by simultaneously considering the interaction of two adjacent Er-ions and the reabsorption loss of the ground-state energy levels.

Deterministic Optical Rogue Waves in Nd:YVO4 Lasers Induced by Near-Degenerate Transverse Modes

We identify the role of transverse modes in causing temporal instability in an all-solid-state laser system and thus propose a new method to trigger deterministic optical rogue waves. Experimental investigations show that the near-degenerate transverse modes with mode frequency difference close to the relaxation oscillation frequency is responsible for the rogue waves. By tilting the reflective mirror, the transverse modes beating frequency decreases and gets close to relaxation oscillation frequency, thus the system exhibits self-pulsing behavior and rarely-arising giant pulses. The statistical distribution of the pulse intensity deviates from the exponential statistics to the L-shaped distributions with long tails, indicating the appearance of optical rogue waves. The conclusion can be extended to other Class-B lasers where the population inversion cannot be eliminated adiabatically.

Quantum correlations across two octaves from combined up- and down-conversion

We propose and analyse a cascaded optical parametric system which involves three interacting modes across two octaves of frequency difference. Our system, combining degenerate optical parametric oscillation (OPO) with second harmonic generation (SHG), promises to be a useful source of squeezed and entangled light at three differing frequencies. We show how changes in damping rates and the ratio of the two concurrent nonlinearities affect the quantum correlations in the output fields. We analyse the threshold behaviour, showing how the normal OPO threshold is changed by the addition of the SHG interactions. We also find that the inclusion of the OPO interaction removes the self-pulsing behaviour found in normal SHG. Finally, we show how the Einstein-Podolsky-Rosen correlations can be controlled by the injection of a coherent seed field at the lower frequency.

Exploring the self-mode locking of the 2 μm Tm:YAG laser with suppression of the self-pulsing dynamic.

A continuous-wave (cw) self-mode-locked Tm:YAG laser at 2015nm is successfully demonstrated by suppressing the self-pulsing behavior. By using rate equations to simulate the laser temporal dynamic, the theoretical analysis indicates that the reabsorption-induced self-pulsing for the Tm:YAG laser can be suppressed with sufficient pump power. It is experimentally confirmed that at absorbed powers higher than 6W, the self-pulsing can be eliminated and a cw self-mode-locked pulse is generated instead of the Q-switched mode locking. At an absorbed power of 6.8W, the output power of the self-mode-locked Tm:YAG laser reaches 1.22W with corresponding pulse duration of 3ps and a repetition rate of 3.376GHz.

Coupled optical microresonators for microwave all-optical generation and processing.

Two coupled microresonator systems with instantaneous nonlinear index can exhibit self-pulsing behavior and could be used as all-optical integrated microwave generators. The frequency of the RF signal is limited by the photon lifetime of the resonators. For oscillations at tens of GHz, semiconductor microdisks with moderate quality factors (≈10(4)) can be used. We show here that using an optimized design the frequency of the generated signal is not sensitive to microresonator resonance frequency fluctuations. Furthermore, it is possible to process the generated microwave signal in the optical domain by cascading integrated optical filters.

Self-pulsing-free continuous-wave operation of an all-fiberized Yb fiber laser

Self-pulsing-free continuous-wave operation of an Yb fiber laser based on an all-fiberized configuration incorporating an additional fiber Fabry–Perot cavity is reported. The fiber Fabry–Perot cavity as a feedback source for lasing allows the fiber laser to operate in stable longitudinal modes resulting in suppression of the self-pulsing behavior. The Yb fiber laser yielded 5.9 W of the stable cw output at 1064 nm for 8.9 W of absorbed pump power corresponding to a slope efficiency of ∼74%. The mechanism for suppression of the self-pulsing and the prospect of further power scaling are discussed.

Characteristics of a propagating, self-pulsing, constricted ‘γ-mode-like’ discharge

Investigations on the self-pulsing operation regime of a modified micro-scaled atmospheric pressure plasma jet (μ-APPJ) are presented. Using a wedge-shaped electrode configuration, a self-pulsing behavior of the device is achieved, which is characterized by the repetitive ignition of a constricted ‘γ-mode-like’ discharge at the gas inlet, which propagates with the gas flow towards the nozzle, where it extinguishes. The ‘γ-mode-like’ feature coexists with the homogeneous alpha-glow. Synchronized voltage/current and optical emission measurements are presented in order to correlate the evolution of electrical quantities such as voltage, current, dissipated power and phase with changes in the discharge structure. First insights are gained into the underlying discharge dynamics responsible for a stable self-sustainment, propagation and extinction of the constricted discharge. The results indicate that processes induced by helium metastables play a major role. Maximal electron densities on the order of ne = 3.2 · 1012 cm−3 and dissipated power of 18.9 W are achieved in this novel operation regime.

Suppression of self‐pulsing in Yb fibre lasers coupled with external Fabry‐Pérot cavity

A simple technique to suppress self-pulsing behaviour in a fibre laser employing a two-mirror Fabry-Perot external feedback cavity is reported. Feedback provided by the external Fabry-Perot cavity enforces the fibre laser to oscillate in the selected longitudinal modes, resulting in robust laser operation without self-pulsing. This technique has been successfully applied to a double-clad Yb fibre laser, yielding a stable continuous-wave output of up to 9.8 W at 1050 nm. The advantages of this technique and the prospects of further improvement are discussed.

Self-Pulsation in a Nonlinear Plasmonic Ring Resonator

We demonstrate the self-pulsing behavior in a nonlinear all-pass plasmonic microring resonator (MRR). The structure consists of a nonlinear metal-dielectric-metal (MDM) plasmonic waveguide and supports antisymmetric, backward propagating slow-modes. The slow modes are coupled to a feedback loop realized from a MDM waveguide, which supports symmetric, forward propagating fast modes. We calculate the temporal response of the system with a finite difference time domain method and show a SP behavior for input fields higher than a critical value. We perform a semi-quantitative description of the self-pulsation by solving iteratively, in time domain, the Ikeda equation of an all-pass MRR presenting a nonlinear phase shift. For this system, the pulses have a period of 300 fs with a modulation factor of 0.3. This configuration could be applied as a source of continuous wave terahertz radiation or as an optical clock in highly integrated plasmonic circuits.

Investigations on the afterglow of a thin cathode discharge in argon at atmospheric pressure

A thin cathode discharge consists of two electrodes separated by a dielectric layer with a thickness of ca 100 µm. The shape of the anode can be chosen arbitrarily, while the thickness of the cathode is also about 100 µm. Through this ‘sandwich’, a hole with a diameter of 200 µm is drilled. When such a device is operated at pressures of several hundred hectopascals, it shows a self-pulsing behaviour in which high electron densities of several 1016 cm−3 are reached. Electrical measurements showed that this can be explained by the repeated ignition of a short-living spark discharge. Due to the high pressure and the related high collision frequencies, the afterglow of this discharge was expected to last several tens of nanoseconds. Instead, lifetimes of several hundred nanoseconds were observed. In order to identify the mechanisms responsible for this long living afterglow, a kinetic model of the afterglow was developed. As a result, Penning ionization, superelastic collisions with both atoms in excited states and excimers were found to play a crucial role in the production and heating of electrons.

Self-pulsing and chaos in short chains of coupled nonlinear microcavities

We examine time-domain instabilities in short chains of coupled Kerr-nonlinear resonators. We show that a large parameter region of self-pulsing or chaotic behavior can be realized. We give a detailed description of the possible states in systems with two and three cavities, using phenomenological coupled-mode equations and rigorous simulations. A particular geometry can exhibit a rich range of dynamics, dependent on input conditions. A clear link with the linear transmission properties is shown. This system complements the studies of the nonlinear Bragg reflector and electrons in a nonlinear lattice. Unlike the Bragg grating case, we observe wide detuning ranges that exhibit self-pulsing without first going through a bistable transition.

Dissipative light bullets in an optical parametric oscillator

When chromatic dispersion operates together with two-dimensional diffraction, the degenerate optical parametric oscillator exhibits three-dimensional (3D) localized structures in a regime devoid of Turing or modulational instabilities. They consist of single lamella, cylinder, or light drops. These 3D structures are generated spontaneously from a weak random noise. We construct 3D bifurcation diagrams associated with these structures. We show that a single cylinder and a single light drop exhibit an overlapping domain of stability. Finally, for a large input intensity, we identify a self-pulsing behavior affecting the stability of 3D localized structures.

Self-pulsing behavior in a Nd:YAG ion-implanted waveguide laser

We report the observation of sustained self-pulsing behavior of the output of a channel waveguide Nd:YAG laser under a CW pump. The duration and repetition rate of the pulses observed were dependent on the pumping power. At the maximum pumping power available (44 mW of absorbed power), the pulse duration was around ∼40 ns, and the pulse repetition rate was ∼1.4 MHz. We discuss the possible physical mechanisms that generate the pulsing behavior, and their implications on the observed pump-power dependence of the repetition rate.

Theoretical modelling of the effect of photon lifetime on the output dynamics of Er-doped distributed feedback fibre lasers

By employing a simple model of describing three-level lasers, we have theoretically investigated the effect of photon lifetime on the output dynamics of Er-doped distributed feedback fibre lasers. And based on the theoretical analysis we have proposed a promising method to suppress self-pulsing behaviour in the fibre lasers.

Self-pulsing in a double-clad ytterbium fiber laser induced by high scattering loss

The characteristics of self-pulsing behavior in a double-clad ytterbium-doped fiber laser are presented. The self-pulsing generates a supercontinuum spectrum that mostly propagates through the multimode structure of the fiber. In fact, 90% of the total power, including the fundamental laser wavelength and the generated supercontinuum spectrum, propagates trough the first cladding. The experimental evidence indicates that the main mechanism for the self-pulsing is the high scattering loss in the double-clad fiber.

Degenerate intracavity parametric processes with injected signal

A common way of increasing the efficiency of optical frequency conversion processes is by the injection of a coherent signal at the desired frequency. We study the efficiency of this method and its effect on the quantum statistics of the fields by performing theoretical analyses of the intracavity parametric χ (2) processes of second harmonic generation and degenerate optical parametric oscillation with injected signal fields. We find that the threshold behaviour of the optical parametric oscillator with an injected signal field gives further insight into the normal threshold behaviour, considered as a limiting case as the signal field goes to zero. An injected signal is also shown to change the critical points of the systems, which define the region where the maximum of noise suppression and other quantum effects may be expected. We also investigate the self-pulsing behaviour of second harmonic generation, showing how an injected signal can affect the oscillations. We show the process of second harmonic generation can be blocked by an injected signal of the appropriate intensity, effectively removing the crystal from the cavity.

Suppression of self-pulsing behavior in erbium-doped fiber lasers with a semiconductor optical amplifier

We experimentally demonstrate that the stability of cw and mode-locked erbium-doped fiber ring lasers can be improved significantly with a semiconductor optical amplifier (SOA) inside the cavity. The fast saturable gain of the SOA suppresses significantly the self-pulsing that is due to ion pairs in the erbium-doped fiber, which acts as a saturable absorber. A linear stabilization analysis of the laser system agrees with our experimental results.