In this paper, based on a linear polarized, white noise signal (WNS) phase modulated all fiber amplifier, the self-pulsing characteristics of polarization maintaining (PM) amplifier at the different phase modulation parameters with the same linewidth are analyzed experimentally. It is demonstrated that the self-pulsing threshold is closely related to the details of the modulation spectrum affected by the frequency and the signal power of the WNS, and it is near impossible to calculate by the linewidth. Besides, by suppressing the self-pulsing effect, we obtain an output of 2009 W with a linewidth of 22 GHz. The polarization extinction ratio (PER) is larger than 15 dB, and the M2 is lower than 1.2.
In this paper, we demonstrate 47 GHz linear polarized fiber amplifier injected by a simple fiber oscillator laser seed source with narrow linewidth and near diffraction-limited beam quality. Output powers of 419 W, 778 W, and 1107 W are achieved with 3dB linewidth of 26.5 GHz, 41 GHz, and 47 GHz, respectively. The M2 is 1.25 in the x-direction and 1.23 in the y-direction at the maximum laser power, respectively. The measured PER is large than 96% during the power scaling process. However, The SRS is observed when the laser power is 1107, the SNR is about 47dB, which means that the SRS effect has become a serious limitation for further power scaling of such PM-amplifier seeded by the fiber oscillator laser seed source.
The seed source with spectral linewidth broadening via phase modulation is potential to achieve the higher output power with effective SBS suppression. However, self-pulsing from the amplifier output is harmful. In this work, we study the self-pulsing characteristics in a long single-mode fiber with lower self-pulsing threshold instead of the high power amplifier. We provide a powerful experimental support for the self-pulsing mechanism in high-power narrow-linewidth fiber lasers, which is important for further output power scaling.
A high energy, high beam quality short-pulse diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with two amplifier stages is demonstrated. The two-rod birefringence compensation was used as beam quality controlling methods, which presents a short-pulse energy of 40 mJ with a beam quality value of M2 = 1.2 at a repetition rate of 400Hz. The MOPA system delivers a short-pulse energy of 712.5 mJ with a pulse width of 12.4 ns.The method of spherical aberration compensation is improved the beam quality, a M2 factor of 2.3 and an optical-to-optical efficiency of 27.7% is obtained at the maximum laser out power.The laser obtained 1.4J out energy with polarization integration.
The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.
Tapering will raise the signal loss in an end-pumped (N+1)×1 type combiner. In this paper, the Thermally Expanded Core (TEC) technique is used in the signal loss optimization experiment with the tapering ratio of the pump combiner is 0.6. The experimental results indicate that the coupling efficiency of the 1.55μm signal light increases from 81.1% to 86.6%, after being heated 10 minutes at the homo-waist region of the tapered signal fiber with an 8mm wide hydroxygen flame. Detail analysis shows that the TEC technique can both reduce the loss of the LP01 mode and the LP11 mode in the signal fiber.
For powers exceeding a threshold the spectral broadening in fiber amplifiers becomes a significant challenge for the development of high power narrow bandwidth fiber lasers. In this letter, we show that the spectral broadening can be partly caused by four-wave mixing(FWM) process in which the power of the central wavelength would transfer to the side ones. A practical FWM induced spectral broadening theory has been derived from the early works. A numerical model of fiber amplifier has been established and FWM process has been added to the model. During the simulation process, we find that when a 10 GHz, several watts narrow bandwidth laser is seeded into a few modes fiber laser amplifier, the FWM induced spectral broadening effect might continually increase the FWHM of the spectra of the continuum laser to 100 GHz within the amplification process to several hundred watts which has been convinced by our experiments. Some other results have also been analyzed in this paper to complete the four-wave mixing induced spectral broadening theory in fiber amplifiers.