Investigations of a 1018 nm gain-switched Yb-doped fiber oscillator.

In this paper, we investigate a 1018 nm gain-switched ytterbium-doped fiber oscillator at a low repetition rate in terms of theory and experiment. Theoretically, a numerical model applicable to a 1018 nm gain-switched ytterbium-doped fiber laser was established. The influence of the pump peak power and active fiber lengths on the 1018 nm gain-switched ytterbium-doped fiber laser was numerically simulated. Experimentally, a compact 1018 nm all-fiber-structured pulsed laser oscillator is constructed, in which a pulse width of 110 ns and a single-pulse energy of 0.1 mJ were obtained. Moreover, the experimental results are in agreement with the numerical simulation ones. To the best of our knowledge, this is the first time that gain-switching technology has been applied to 1018 nm fiber lasers to generate nanosecond pulsed lasers. The model and experimental results can provide a reference for the engineering design of the same type of low repetition rate fiber lasers below the kilohertz level.

All-fiber hectowatt radially polarized laser system.

A high-power all-fiber radially polarized laser system is demonstrated, in which an integrated nanograting mode convertor (S-wave plate) is used for the generation of radially polarized beam. Experimentally, a 1-W radially polarized beam was used as the seed laser, whose mode purity and mode extinction ratio (MER) were 96.5% and 98.3%, respectively. A single-stage few-mode fiber amplifier was employed to boost the 1-W seed laser to an average power of 113.2 W, when the pump power was 160 W. The corresponding slope efficiency and beam quality factor (M2) were approximately 72% and 2.3%, respectively. Moreover, the mode purity and MER of the amplified radially polarized laser were measured to be 95.7% and 97%, respectively. To the best of our knowledge, this is the highest output power from an all-fiber radially polarized laser system without obvious degradations of the mode purity and MER.

Nonlinear spectral compression in high-power narrow-linewidth polarization maintaining fiber amplifiers for SBS suppression.

In this paper, we propose a method for narrowing the spectrum in high-power narrow-linewidth polarization-maintaining (PM) fiber amplifiers and investigate its potential for suppressing the stimulated Brillouin scattering (SBS). In this method, in addition to common phase modulation to suppress SBS, precisely designed amplitude modulation is induced to generate self-phase modulation in a high-power PM fiber amplifier. In this co-modulation way, the spectrum can be gradually compressed along the fiber. Compared to phase modulation alone or fiber-Bragg-gratings (FBGs) based narrow-linewidth fiber oscillator schemes, in which the spectrum remains the same or broadens, this scheme can achieve a higher SBS threshold for the same output spectral linewidth. Experiments on a ∼ 3 kW peak power quasi-continuous wave (QCW) fiber amplifier show that the co-modulation scheme can compress the spectrum from 0.25 nm to 0.084 nm as output peak power increases from 13 W to 3.2 kW and enhances the SBS threshold by ∼1.7 times compared to traditional FBGs-based fiber oscillator schemes, and by ∼1.4 times compared to common phase modulation schemes. This co-modulation scheme has the potential for mitigating SBS in high-power fiber amplifiers.

Coherent beam combining of cylindrical vector beams for power scaling.

Coherent beam combining (CBC) of cylindrical vector beams (CVBs) based on an active phase controller is proposed and demonstrated. Experimentally, two pieces of spatially variant wave plates (S-wave plate) were employed as vector mode convertors for two individual 1064-nm fiber amplifier channels. When the system was in a closed loop, a combined output power of 10 W and a CBC efficiency of higher than 94% were achieved, for both TM01 (radially polarized) mode and TE01 (azimuthally polarized) mode cases. Moreover, the laser system showed a high vector mode purity, which was measured to be ∼97.4% (TM01) and ∼97.3% (TE01). To the best of knowledge, this could possibly be the first demonstration of CBC of CVBs, which paves the way for power scaling of CVBs.

Linearly polarized fiber amplifier with narrow linewidth of 5†kW exhibiting a record output power and near-diffraction-limited beam quality.

In this work, a narrow-linewidth polarization-maintaining (PM) all-fiber amplifier with near-diffraction-limited beam quality and record output power is presented. First, a 4.45-kW PM fiber amplifier with a 3-dB linewidth of 0.08 nm and root mean square (rms) linewidth of 0.22 nm is achieved based on optimized phase modulation. However, the sideband of the spectrum broadens significantly during the amplification process, which is mainly caused by the additional intensity variation of the injected signal. Meanwhile, an up to 5.04-kW linearly polarized fiber laser with a relatively stable spectral bandwidth is achieved by effectively suppressing spectral broadening. At the maximum output power, the rms linewidth is 0.2 nm, the beam quality factor M2 is less than 1.3, the polarization extinction ratio (PER) is 16.5 dB, and the signal-to-noise ratio (SNR) is approximately 53 dB. The further power scaling of the amplifier is mainly limited by the pump power. To the best of our knowledge, this is the maximum output power of a narrow linewidth linearly polarized fiber amplifier to date.

3.25 kW all-fiberized and polarization-maintained Yb-doped amplifier with a 20 GHz linewidth and near-diffraction-limited beam quality.

In this paper, we demonstrate a high-power, narrow-linewidth, polarization-maintaining fiber amplifier with near-diffraction-limited beam quality. By optimizing the phase modulation signal, a nearly top-hat-shaped spectrum was generated for self-pulsing suppressing. That results in doubling the self-pulsing threshold we got from conventional white noise signal phase modulation with the same optical linewidth. Based on an optimized signal and a high power, polarization-maintaining, counter-pumped fiber amplifier, we obtain a 3.25 kW narrow-linewidth linearly polarized laser output with a linewidth of ∼20GHz, the polarization extinction ratio is about 15 dB, and the M2 is less than 1.22 at the maximum output power. To the best of our knowledge, this is the first demonstration of a narrow-linewidth, linear polarization, all-fiber amplifier with 3.25 kW laser output.

1.5 kW polarization-maintained Yb-doped amplifier with 13 GHz linewidth by suppressing the self-pulsing and stimulated Brillouin scattering.

In this work, we study the characteristics of self-pulsing in a polarization-maintained fiber amplifier operated with different linewidths based on white noise source phase modulation. It indicates that the self-pulsing is almost simultaneous with the stimulated Brillouin scattering process, and its threshold is increasing near-linearly with the linewidth. By optimizing the laser structure, the threshold of self-pulsing increases by a factor of 1.5. We demonstrate a high-power linear polarization and all-fiberized amplifier with narrow linewidth and near-diffraction-limited beam quality. The output power scales to 1.5 kW with the pumping efficiency of 83%. The full width at half-maximum linewidth was measured to be 13 GHz. The polarization extinction ratio was larger than 13 dB. The beam quality M2 was about 1.14 at the maximum laser power.

Self-pulsing in kilowatt level narrow-linewidth fiber amplifier with WNS phase-modulation.

The self-pulsing phenomenon in kilowatt level narrow-linewidth fiber amplifiers with white noise source (WNS) phase-modulation is observed experimentally. It possesses the obvious threshold of the pump power and prevents the narrow-linewidth fiber lasers from further power scaling. The experimental study shows that known explanations are not applicable here and indicates that occurrence of self-pulsing is closely related to Stimulated Brillouin Scattering (SBS) process. The theoretical discussion reveals that the spikes in the modulated spectrum are the critical factor that SBS threshold is lower than the theoretical estimation. The 1+1 dimensional SBS model analysis predicts that self-pulsing originates from forward second order Stokes pulses, which is in good qualitative agreement with the experimental data.

6.5 GHz linearly polarized kilowatt fiber amplifier based on active polarization control.

The polarization-dependent transverse mode properties in a few-mode nonpolarization-maintaining (non-PM) fiber amplifier with PM fiber laser seeding are investigated. In addition, the effect of spectral broadening on degeneration of the degree of polarization (DOP) is studied. The experimental results show that a high DOP can be achieved if the output laser of the non-PM fiber amplifier has good beam quality and a narrow linewidth without any broadening. A kilowatt (kW) fiber laser system with PM fiber seed and a non-PM fiber amplifier is demonstrated with the DOP at around 90% by controlling the beam quality and spectrum broadening. With the active polarization control technique, the system can achieve a kW linearly polarized output with 6.5 GHz linewidth. The M2 is around 1.1, and the polarization extinction ratio is about 14.5 dB. Unfortunately, the random self-pulsing issue arose when the output power exceeded 1093 W. We believe that a higher power of linearly polarized laser output based on adaptive polarization control in a non-PM fiber amplifier will be obtained if the self-pulsing issue is solved.

DFB laser based on single mode large effective area heavy concentration EDF.

A π phase shifted distributed feedback (DFB) laser based on single mode large effective area heavy concentration erbium-doped fiber (EDF) has been demonstrated. The homemade EDF was fabricated by the modified chemical-vapor deposition (MCVD) technique, and the 13cm long π phase shifted fiber grating was written in the intracore of the EDF. The erbium-doped concentration is 4.19 × 10(25) ions/m(3), the mode field diameter of the fiber is 12.2801 um at 1550 nm, the absorption coefficients of the fiber are 34.534 dB/m at 980 nm and 84.253 dB/m at 1530 nm. The threshold of the DFB laser is 66 mW, and the measured maximum output power is 43.5 mW at 450 mW pump power that corresponding to the slope efficiency of 11.5%. The signal-to-noise ratio (SNR) of the operating laser at 200 mW input power is 55 dB, and the DFB laser has a Lorentz linewidth of 9.8 kHz at the same input pump power.