Laser Instability Research Articles

Comparison of the polarization dynamics of three different gain fibers in a continuous wave laser oscillator

We experimentally investigate the polarization dynamics of three kinds of gain fiber, the single-polarizing photonic crystal fiber, double clad polarization-maintaining large mode area fiber and double clad large mode area nonpolarizing fiber, in a continuous wave fiber laser oscillator under different initial polarization directions and pumping powers. The comparison results demonstrate that there are some distinguishable differences for the polarization evolutions inside the three kinds of active fibers. The polarization operation status of the fiber oscillator strongly depends on not only the gain fiber’s kinds, but also the length of the fiber, initial polarization directions and pumping power. The results suggest that polarization instability in a fiber laser should be an inherent reason for the higher operation noise.

Direct experimental observation of stimulated thermal Rayleigh scattering with polarization modes in a fiber amplifier

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.

Continuous 1052, 1064 nm dual-wavelength Nd:YAG laser

Dual-wavelength lasers are usually obtained through balancing the net gain of the two oscillating lines. Competition between transitions 1052nm, 1061nm and 1064nm is utilized to realize a continuous wave 1052 and 1064nm dual-wavelength Nd:YAG laser firstly in this paper. A specially designed Fabry-Perot band-pass filter is exploited as output coupler to control the thresholds of the oscillating wavelengths. The maximum power of the dual-wavelength laser is 1.6W and the slope efficiency is about 10%. The power instability of the output dual-wavelength laser is smaller than ±4% in half an hour. The mechanism presented in this paper may provide a new way to obtain dual-wavelength lasers.

Intrinsic performance-limiting instabilities in two-level class-B broad-area lasers

The present paper is concerned with the analytical and numerical investigation of the transverse spatio-temporal instabilities in two-level broad-area lasers for the specific class-B case. We show that the two-level class-B broad-area laser tends to naturally operate in the filamentary state. This is revealed to be provided with two causes. First of them is related with the homogeneous output profile being intrinsically unstable due to the traveling-wave instability, independently from the boundary conditions. Secondly, high sensitivity to the boundaries of the pumping region was found for the commonly used top-hat-like profile leading to boundaries-induced filamentary dynamics. Spatio-temporal properties of both instability mechanisms are studied and their effects on the resulting laser dynamical behavior are analyzed.

Precise manipulation on spike train of uneven duration or delay pulses with a time grating system.

In this paper, we proposed a time grating system to achieve spike train of uneven duration or delay (STUD) pulses, and theoretically study their features under various modulation conditions. This time grating scheme, which is a temporal analogy of spatial grating, introduces great degree of freedom for controlling the output pulse characteristics (pulse width, repetition rate, pulse shape, etc.) through simply tuning the electronics elements and the programmable phase modulation function. The narrowest pulse width is highly determined by the modulation parameters and the branch number N, and the numerically obtained value is around tens of femtoseconds in the current case. When super-Gaussian pulses are modulated with an optimized and modified trapezoidal function, the pulse rising/falling edge can be greatly compressed to form a clean nearly-square wave (with edges less than 10 fs). STUD pulses generated with this time grating system have high-degree controllability and are very beneficial for suppressing parametric instabilities in laser driven inertial confinement fusion.

Brillouin Instability in Fiber Lasers Doped By Power

With the emergence of rare-earth doped fibers, and especially double-clad fibers, there is a renewed interest in Brillouin effect. First of all, the amplification of a continuous signal in a rare-earth doped fiber amplifier can generate high enough intensities to excite Brillouin effect and then to create a backscattered stokes wave. Such back-reflection is detrimental for amplifier applications and consequently it has been studied theoretically and experimentally. Recently, the low frequency self-pulsing instability resulting from Brillouin backscattering has been theoretically modelled [1]. Our main objective is to present a general model allowing to explain the origin of the dynamic instability arising in a fiber lasers as a consequence of Brillouin effect. The effect of Brillouin back scattering is theoretically analysed by two-coupled modes laser model. We consider the Fabry-Perot fiber laser cavity. The rich and complex dynamic behaviours are observed. In particular the quasi periodic dynamic is identified and studied. Keywords : Stimulated Brillouin Scattering; high power fiber lasers, instabilities, quasi periodic.

Study of Wavelength Dependence of Mode Instability Based on a Semi-Analytical Model

We present theoretical study of wavelength dependence of mode instability (MI) in high power fiber lasers, which employs an improved semi-analytical theoretical model. The influence of pump / seed wavelength and photodarkening on threshold has been studied. The results indicate promising MI suppression through pumping or seeding at an appropriate wavelength. Small amounts of photodarkening can lead to significant impact on MI.

Simultaneously reducing the intensity and frequency noise of single-frequency phosphate fiber laser

We demonstrate an optoelectronic feedback method to simultaneously reduce the intensity and frequency noise of a single-frequency phosphate fiber laser for the first time. Based on the fact that the intensity and frequency noise in this laser mainly originate from the pump power fluctuations, we implement a simple and compact feedback loop and achieve a maximum noise reduction of over 20 dB for the intensity noise at frequencies of 0.3–3 kHz and 10 dB for the frequency noise at around 1 kHz. In addition, the laser instability is improved from ±1.7% to ±0.1%, and the laser line shape is observed to be compressed at the central part.

Effect of dynamical non-neutrality on the modulational instability of laser propagating through hot magnetoplasma

In this work, modulational instability of a laser pulse in a hot magnetized plasma is investigated. Nonlinear relativistic equation, describing the amplitude evolution of a laser with finite longitudinal and transversal structure, is obtained. Taking into account the plasma dynamical non-neutrality caused by the ponderomotive force, the growth rate of the modulational instability is derived. Effect of the pulse length on the instability growth rate is investigated.

Simultaneously Suppressing Low-Frequency and Relaxation Oscillation Intensity Noise in a DBR Single-Frequency Phosphate Fiber Laser**Supported by the National High-Technology Research and Development Program of China under Grant Nos 2013AA031502 and 2014AA041902, the National Natural Science Foundation of China under Grant Nos 11174085, 51132004, and 51302086, the Guangdong Natural Science Foundation under Grant Nos S2011030001349 and S20120011380, the China National Funds

Effective multiple optoelectronic feedback circuits for simultaneously suppressing low-frequency and relaxation oscillation intensity noise in a single-frequency phosphate fiber laser are demonstrated. The forward transfer function, which relates the laser output intensity to the pump modulations, is measured and analyzed. A custom two-path feedback system operating at different frequency bands is designed to adjust the pump current directly. The relative intensity noise is decreased by 20 dB from 0.2 to 5kHz and over 10 dB from 5 to 10 kHz. The relaxation oscillation peak is suppressed by 22 dB. In addition, a long term (24 h) laser instability of less than 0.05% is achieved.

Using pipe with corrugated walls for a subterahertz free electron laser

It has been noted in the past, in the study of the wall-roughness impedance, that a metallic pipe with corrugated walls supports propagation of a high-frequency mode that is in resonance with a relativistic beam. This mode can be excited by a beam whose length is a fraction of the wavelength. In this paper, we study another option of excitation of the resonant mode in a metallic pipe with corrugated walls---via the mechanism of the free electron laser instability. This mechanism works if the bunch length is much longer than the wavelength of the radiation.

Cross-Phase Modulation Instability in Mode-Locked Laser Based on Reduced Graphene Oxide

Cross-phase modulation instability (XPMI) is experimentally observed in a fiber ring cavity with net normal dispersion and mode-locked by long fiber taper. The taper is deposited with reduced graphene oxide, which can decrease the threshold of XPMI due to the enhanced nonlinearity realized by 8-mm evanescent field interaction length and strong mode confinement. Experimental results indicate that the phase matching conditions in two polarization directions are different, and sidebands with different intensities are generated. This phase matching condition can be satisfied even the polarization state of the laser varies greatly under different pump strengths.

Performance Optimization and Instability Study in Ring Cavity Quantum Cascade Lasers

An optimization study of quantum cascade lasers (QCLs) considering the laser instability condition is performed. To model current transport, the Pauli master equation is solved using a Monte Carlo approach. The effects of saturable absorber and pumping strength on the instability threshold are investigated. A particle swarm optimization algorithm is applied to increase the laser optical gain. A large optical gain below the instability threshold is achieved for optimized QCL designs. To analyze the optimized structure, a numerical calculation based on the Maxwell-Bloch equations is performed. The results indicate side-mode instabilities due to Risken-Nummedal-Graham-Haken-like instability.

Qubit fidelity of a single atom transferred among the sites of a ring optical lattice

We demonstrate the transfer of a single atom in a ring optical lattice with the aid of an auxiliary moving tweezer and investigate the influences on fidelity of the qubit encoded in the atom. When the tweezer has deeper trap depth and moves across the lattice, it is observed that an atom in one site follows the movement. The transfer efficiency of one atom to the destination site reaches up to 95%. This scheme is suitable for scalable quantum registers because of no influence on the other sites. We obtain atomic qubit fidelity during the transfer process by using quantum state tomography. Extracted fidelity indicates that the eigenstate is well preserved, while the superposition state is influenced. In combination with spin-echo measurement, dephasing mechanisms in this process are analyzed and discussed in detail. Loss of qubit fidelity is found to result from heating effects induced by this process and pointing instabilities of the trap laser. Our results pave the way for quantum computation with single atoms trapped in a scalable optical lattice.

Modal instability in high power solid-state lasers with an unstable cavity

A phenomenon of modal instability is investigated theoretically in some high power solid-state lasers with an unstable cavity. This modal instability is caused by a thermo-optic-thermo feedback coupled with an asymmetrical laser oscillation inside some unstable cavities. Numerical investigations indicate that this instability can appear in all lasers with Nd3+-doped disk scheme, Yb3+-doped disk scheme and direct-liquid-cooled slab scheme, but the physical mechanisms of optic-thermo feedback in the three schemes are completely different. It is shown that the modal instability can appear within a special power range only in the Nd- or Yb-doped disk lasers due to certain saturation phenomenon, but exhibits a power threshold in the direct-liquid-cooled slab lasers. Therefore different strategies to mitigate the modal instability in different lasers are suggested. Finally, the presence of transverse modal instability in the Nd:YAG thin-disk laser is confirmed qualitatively by an experiment.

Reflectivity of transient Bragg reflection gratings in fiber laser with laser-wavelength self-sweeping.

We present a method for the estimation of the reflection spectra of transient gratings in rare-earth doped fiber lasers having a self-sweeping of laser wavelength. We show that high reflectivities of several tens of percent can be achieved. An example of this is demonstrated through the use of an experimental Yb-doped Fabry-Perot fiber laser. The gratings' spectra are highly asymmetric due to the apodization of the refractive index modulation. The importance of the self-sweeping regime for triggering self-Q-switched laser instabilities is discussed.

Phase instability in semiconductor lasers.

For many years, the apparent absence of a phase instability has characterized lasers as peculiar nonlinear oscillators. We show that this unusual feature is solely due to the approximations used in writing the standard models. A new, careful derivation of the fundamental equations, based on codimension 2 bifurcation theory, shows the possible existence of dynamical regimes displaying either a pure phase instability, or mixed phase-amplitude turbulence. A comparison to existing experimental results convincingly shows that the Benjamin-Feir instability, common to all nonlinear wave problems, is a fundamental, satisfactory interpretation for their deterministic multimode dynamics.

Calibration of the straightness and orthogonality error of a laser feedback high-precision stage using self-calibration methods

An ultra high-precision 3-DOF air-bearing stage is developed and calibrated in this study. The stage was developed for the transportation of a glass or wafer with x and y following errors in the nanometer regime. To apply the proposed stage to display or semiconductor fabrication equipment, x and y straightness errors should be at the sub-micron level and the x–y orthogonality error should be in the region of several arcseconds with strokes of several hundreds of mm. Our system was designed to move a 400 mm stroke on the x axis and a 700 mm stroke on the y axis. To do this, 1000 mm and 550 mm bar-type mirrors were adopted for real time Δx and Δy laser measurements and feedback control. In this system, with the laser wavelength variation and instability being kept to a minimum through environmental control, the straightness and orthogonality become purely dependent upon the surface shape of the bar mirrors. Compensation for the distortion of the bar mirrors is accomplished using a self-calibration method. The successful application of the method nearly eliminated the straightness and orthogonality errors of the stage, allowing their specifications to be fully satisfied. As a result, the straightness and orthogonality errors of the stage were successfully decreased from 4.4 μm to 0.8 μm and from 0.04° to 2.48 arcsec, respectively.

Observation of mode instability and coherence collapse in a single-frequency polarization-maintaining fiber ring laser

Mode instability acts as a common feature in single-frequency fiber ring lasers. The mechanism of coherence collapse by mode instability is theoretically analyzed and demonstrated with an unbalanced fiber Michelson interferometer utilizing phase modulation, which is illuminated by a single-frequency erbium-doped fiber ring laser. Multiform mode instability phenomena accompanied with coherence collapse are observed and discussed in detail by tracing the dynamics of the interference fringe visibility. The results show that mode instability would introduce extra phase noises like a false alarm to interferometric fiber optic sensing systems.

A stable 2 μm passively Q-switched fiber laser based on nonlinear polarization evolution

A passively Q-switched thulium-doped fiber (TDF) laser based on the nonlinear polarization evolution technique was demonstrated with the central wavelength of 1898.4 nm. With the increasing pump power, the pulse repetition frequency of the Q-switched TDF laser from 87.6 to 110.1 kHz was achieved, while the corresponding pulse duration was changed from 1171 to 785.7 ns. The power instability of the TDF laser was measured to be about ±1.5% during 8 h. In addition, the mode-locked phenomenon was also observed in our all-fiber TDF laser by carefully adjusting the polarization controllers.