The selector-free single-frequency operation of an Yb-doped fiber laser with scanning in the range of ?20?nm is demonstrated. The frequency and intensity evolution is shown to be driven by a self-induced phase grating in the active fiber defined by gain saturation in a standing-wave. A theory has been developed that describes well the main features of the experiment and provides possibilities for optimization of laser parameters. Perspectives for utilizing the self-scanned laser in fundamental studies and practical applications are discussed.

Self-scanned single-frequency operation of a fiber laser driven by a self-induced phase grating

The effect of broad-range (16 nm) self-sweeping of a narrow-line (less than 1 pm) Yb-doped fiber laser has been demonstrated experimentally. It is found that the effect arises from the self-sustained relaxation oscillations. As a result, the sweeping rate increases as square root of the laser power and decreases with increasing cavity length. Based on these results we propose a model describing dynamics of the laser frequency. The model takes into account the effects of gain saturation at the laser transition and spatial hole burning in the self-pulsing regime.

Broad-range self-sweeping of a narrow-line self-pulsing Yb-doped fiber laser

We present a brand-range self-sweeping Tm-Ho co-doped fiber laser experimentally. The laser’s center wavelength sweeps periodically around 1.9 μm with self-sweeping range of 4 nm ~17 nm, and the sweep rate can be changed in the range of 0.4 nm/s ~1.5 nm/s. The sweep range increases and the sweep rate declines when the pump power rises.

Tm-Ho co-doped all-fiber brand-range self-sweeping laser around 1.9 μm.

Self-swept erbium fiber laser emitting around 1.56 μm is reported in detail. Both sweep directions were registered: pointing toward longer and shorter wavelengths, redshift and blueshift sweeping, respectively. We describe method of determining the direction of the wavelength drift using the monochromator based optical spectrum analyzer. Possible root for this sweeping regime, i.e., the gain modulation along active fiber, is discussed with the help of a simple model calculating the overall cavity gain that can predict the direction of the laser wavelength sweeping.

Self-swept erbium fiber laser around 1.56 μm

Self-induced laser line sweeping of various regimes of sweep direction is reported for an experimental ytterbium fiber laser. The regimes involve sweeping from shorter to longer wavelengths ( nm)—so-called normal self-sweeping; from longer to shorter wavelengths ( nm)—so-called reverse self-sweeping; and a mixed regime in which a precarious balance of the normal and reverse sweeping exists and the sweep direction can change between consecutive sweeps. The regimes of sweeping were selected by changing the pump wavelength only. A detailed explanation of this sweep direction dynamics is presented based on a semi-empirical model. This model also provides a way to predict the sweep direction of fiber lasers based on other rare-earth-doped laser media.

https://iopscience.iop.org/article/10.1088/1612-202X/aa548d/pdf

Reverse spontaneous laser line sweeping in ytterbium fiber laser

The results of an experimental investigation were combined with the published work to show that undamped spikes in the output of multimode pulsed lasers were due to an inhomogeneous depletion of the inverted population in the bulk of an active rod. A quasicontinuous output was obtained from a ruby laser with plane mirrors. It was found that the mechanical and thermal distortions of the resonator, which could suppress the laser action, were essentially misalignments. A new feature of the stimulated emission, called inertia, was observed for a ruby laser with spherical mirrors when the spatial inhomogeneities of the population inversion were smoothed out.

Spike structure of the emission of solid-state lasers

A superregenerative amplification scheme was used to obtain unidirectional stimulated emission from a ruby ring laser under free-oscillation and Q-switched conditions. The duration of external control pulses was either tc > L/c or tc < L/c. The carrier frequency of giant pulses could be tuned within a range of 0.25 nm when the spectrum was 6×10–5 nm wide. Reproducible generation of short high-coherence light pulses was achieved.

Ruby ring laser subjected to an external signal

An experimental investigation was made of a new method for controlling the duration of ultrashort pulses in the widest possible range (for pulses shorter than the resonator length) from the axial period to the reciprocal of the width of the gain band of the active medium. The method ensured complete reproducibility of all the parameters of the output radiation. A ruby laser with a saturable absorber in the resonator emitted pulses of ~35 psec duration when pulses of ~1.5 nsec duration were injected from outside into the resonator (the reduction in the pulse duration thus exceeded a factor of 40). The width of the emission spectrum was then close to the limit for a pulse with an exponential envelope.

Control of the duration of ultrashort pulses emitted from a Q-switched solid-state laser

An analogy is suggested between the emission of radiation from a solid-state laser and the frictional motion of a particle in a potential well. This analogy is used to demonstrate analytically that a system of rate equations which allows only for the spatial inhomogeneity of the population inversion fails to describe the undamped pulsation regime. It is shown that this inhomogeneity makes the emission much more sensitive to perturbations in a real resonator. A comparison of the theoretical and experimental results is used to demonstrate the existence of a mechanism which generates undamped pulsations but is not related to laser instabilites.

K. G. Folin

Papers

Spike structure of the emission of solid-state lasers

Ruby ring laser subjected to an external signal

Control of the duration of ultrashort pulses emitted from a Q-switched solid-state laser

Spatial inhomogeneity of inversion and undamped pulsations of solid-state lasers

Effects of smoothing spatial inhomogeneities of the inversion on the laser dynamics of a neodymium-glass laser with spherical mirrors