Long Huang

414 W near-diffraction-limited all-fiberized single-frequency polarization-maintained fiber amplifier

A high-power 1064 nm single-frequency polarization-maintained fiber amplifier based on an all-fiber master oscillator power amplifier configuration is demonstrated. To mitigate the stimulated Brillouin scattering (SBS) and the mode instability (MI) effect, a polarization-maintained Yb-doped fiber with a high dopant concentration and a 25 μm core diameter is adopted in the main amplifier stage; in addition, step-distributed longitudinal strain is imposed on the active fiber to broaden its effective SBS gain spectrum and further increase the SBS threshold. As a result, a pump-limited 414 W single-frequency fiber laser is obtained without signs of SBS and MI. Experimental results show that the SBS threshold is increased by at least two times. The slope efficiency of the main amplifier is about 80%. The polarization degree is higher than 98% at all the power levels. The beam quality is measured with a M2 of 1.34. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on an all-fiber structure.

High power broadband all fiber super-fluorescent source with linear polarization and near diffraction-limited beam quality.

In this manuscript, a high power broadband superfluorescent source (SFS) with linear polarization and near-diffraction-limited beam quality is achieved based on an ytterbium-doped (Yb-doped), all fiberized and polarization-maintained master oscillator power amplifier (MOPA) configuration. The MOPA structure generates a linearly polarized output power of 1427 W with a slope efficiency of 80% and a full width at half maximum (FWHM) of 11 nm, which is power scaled by an order of magnitude compared with the previously reported SFSs with linear polarization. In the experiment, both the polarization extinction ratio (PER) and beam quality (M(2) factor) are degraded little during the power scaling process. At maximal output power, the PER and M(2) factor are measured to be 19.1dB and 1.14, respectively. The root-mean-square (RMS) and peak-vale (PV) values of the power fluctuation at maximal output power are just 0.48% and within 3%, respectively. Further power scaling of the whole system is limited by the available pump sources. To the best of our knowledge, this is the first demonstration of kilowatt level broadband SFS with linear polarization and near-diffraction-limited beam quality.

Kilowatt-level near-diffraction-limited and linear-polarized Ytterbium-Raman hybrid nonlinear amplifier based on polarization selection loss mechanism

Ytterbium-Raman cascaded oscillators with linearly polarized output are designed and achieved based on polarization selection loss (PSL) mechanism for the first time. The 1120 nm laser cavity is designed with fully non polarization-maintained (NPM) fiber Bragg gratings (FBGs) and NPM active fiber while the 1080 nm laser cavity is designed based on polarization-maintained (PM) FBGs and PM active fiber. By using PSL mechanism in 1080 nm cavity, even with fully NPM 1120 nm cavity, both linear-polarized 1120 nm and 1080 nm lasers are achieved in the output port of the cascaded oscillators. Based on the new designed cascaded seeds, a high power polarization-maintained Yb-Raman hybrid nonlinear amplifier is established for further power scaling of the 1120 nm laser. In the nonlinear amplifier, only 21-meter-long active fiber and 1.5-meter-long passive fiber is used for power transferring from 1080 nm to 1120 nm. Output power of 1181 W is achieved at central wavelength of 1120 nm with the M(2) factor of <1.2 and polarization-extinction ratio (PER) of 18.2 dB. As far as we known, the output power of this all fiber format is the highest one in 1120 nm with linear polarization. This type of high power Yb-Raman nonlinear amplifier design with linear polarization can be further extended to Yb-Raman amplifying the wavelength range of 1100-1200 nm.

608 W average power picosecond all fiber polarization-maintained amplifier with narrow-band and near-diffraction-limited beam quality

In this manuscript, we focus on a narrow-band all fiber polarization-maintained (PM) master oscillator power amplifier (MOPA) configuration seeded by a narrow band gain-switched laser diode for near-diffraction-limited average power scaling in sub-nanosecond pulsed regime by controlling detrimental mode instability (MI) effect. The MI free average power scaling abilities of the MOPA system with different pumping wavelengths are explored experimentally. By pumping the main amplifier with 976 nm laser diodes (LDs), experimental results show that near-diffraction-limited average power is just limited to be about 380 W due to the influence of MI effect. However, by selecting a 915 nm pumping wavelength, a mode-instability free average power of 608 W is achieved with pulse width of ~810 ps, peak power of ~128 kW and pulse energy of 60.8 μJ. At 608 W average power, the 3 dB line-width of the whole MOPA configuration is measured to be about 0.19 nm. The beam quality (M2 factor) and polarization extinction ratio (PER) at maximal output power are measured to be within 1.4 and 13 dB, respectively.

1.5 kW ytterbium-doped single-transverse-mode, linearly polarized monolithic fiber master oscillator power amplifier

A linearly polarized monolithic fiber laser based on a master oscillator power amplifier structure with a master oscillator and a one-stage power amplifier is reported. We design a homemade oscillator based on the theory that, in the coiled gain fiber, the higher modes and the polarized mode of the fundamental mode along the fast axis are suppressed effectively because of their obviously higher bend loss than that of the polarized mode of the fundamental mode along the slow axis. The oscillator operates at 1080 nm, launching a 30 W seed laser with a high polarization extinction ratio of 19 dB into the power amplifier via a mode field adapter. The power amplifier utilizes Yb-doped polarization-maintaining fiber of 20/400  μm, which produces nearly diffraction-limited output power of about 1.5 kW with an optical-optical efficiency of 81.5% and a polarization extinction ratio of 13.8 dB. Both the M(x)² factor and the M(y)² factor of the collimated beam are measured to be about 1.2. The spectral width of the output power is broadened approximately linearly, and the full width at half maximum of the spectrum at the maximum output power is about 5.8 nm. It is known as the highest linearly polarized output power to the best of our knowledge.

Experimental investigation of thermal effects and PCT on FBGs-based linearly polarized fiber laser performance.

We experimentally study the impacts of thermal effects and polarization crosstalk (PCT) on the performance of FBGs-based linearly polarized all-fiber laser. The mechanism that the thermal effects and PCT influence the performance of the laser is analyzed. Thermally-dependent reflection peaks of polarization maintaining (PM) fiber Bragg gratings are revealed to be the prime reason why temperature influences spectrum, output power, and polarization property of the laser. The PCT would also influence the performance of the laser seriously in the case of mismatched angle even with effectively overlapped spectrum. It is revealed experimentally that stable linearly polarized output can be obtained if a certain pair of aligned principal axes of PM FBGs is not only spectrally overlapped but also strictly angle matched. Further, we point out that accurate temperature control and careful angle match are essential for stable linearly polarized output and even possible power scaling further.

A high-power LD-pumped linearly polarized Yb-doped fiber laser operating at 1152 nm with 42 GHz narrow linewidth and 18 dB PER

We demonstrate a high-power 1152 nm narrow-linewidth linearly polarized fiber laser based on a commercial polarization-maintaining double cladding Yb-doped fiber and cladding-pump regime at 976 nm. By carefully selecting the parameters of the cavity and heating the gain fiber, a maximum output power of 13 W with a slope efficiency of ~45% is obtained, with the amplified spontaneous emission suppressed more than 35 dB lower than the signal wavelength. The polarization extinction ratio and 3 dB linewidth at the maximum output power are 18 dB and 0.14 nm (~42 GHz) respectively, while no spectral broadening and polarization degradation are observed in the process of power scaling, which is an attractive result for some special applications, such as frequency doubling.

434 W all-fiber linear-polarization dual-frequency Yb-doped fiber laser carrying low-noise radio frequency signal

We demonstrate a high power dual-frequency linear-polarization fiber laser that carries radio frequency signal. Such fiber laser is based on an all-fiber master oscillator power amplifier configuration that consists of a dual-frequency seed laser and three-stage amplifiers. The dual-frequency seed laser is constructed by recombining two beams that are split from a single-frequency linearly-polarized laser. One beam has initial frequency and the other beam is modulated by an acoustic-optical modulator to have a frequency shift of 150 MHz. Then the radio frequency signal of 150 MHz is carried on the laser due to the beat frequency of these two beams. In the main amplifier, a piece of polarization maintaining large-mode-area fiber with short length is used to combine the SBS suppression with high power amplification. As a result, the dual-frequency laser is amplified to 434 W without the occurrence of SBS. The slope efficiency is 81.3%. The polarization degree of the laser and the modulation depth of the optically carried radio frequency signal are both well maintained during the amplification process. Besides, a high signal-noise-ratio of above 75 dB is realized, which demonstrates the low-noise property of the optically carried radio frequency signal. To the best of our knowledge, this is the highest reported output power of the optically carried radio frequency signal.

Diode-Pumped 1178-nm High-Power Yb-Doped Fiber Laser Operating at 125

We demonstrate a high-power Yb-doped fiber laser operating at 1178 nm based on commercial silica double-cladding fiber and laser diode (LD) pumping regime at 976 nm. It is experimentally proved that heating the active fiber is an efficient way to suppress amplified spontaneous emission (ASE) and to increase the parasitic lasing threshold in long-wavelength Yb-doped fiber lasers. By adopting output coupling fiber Bragg grating (OC FBG) with high reflectivity and heating the Yb-doped fiber to 125 °C, a record output power of 31 W is obtained, and the corresponding slope efficiency is ~44% with respect to the pump power. To the best of our knowledge, it is the highest output power of Yb-doped fiber lasers, which adopt commercial double-cladding active fiber and operate in the range beyond 1160 nm. Furthermore, it is first experimentally proved that the parasitic lasing threshold is related to the power of the backward ASE and the length of the active fiber.

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Random distributed feedback fiber lasers (RDFFL) are now attracting more and more attentions for their unique cavity-free, mode-free and structural simplicity features and broadband application potentials in many fields, such as long distance sensing, speck free imaging, nonlinear frequency conversion as well as new pump source. In this talk, we will review the recent research progresses on high power RDFFLs. We have achieved (1) More than 400 W RDFFL with nearly Gaussian beam profile based on crucial employment of fiber mismatching architecture. (2) High power RDFFL with specialized optical property that include: high power narrow-band RDFFL, hundred-watt level linearly-polarized RDFFL, hundred-watt level high-order RDFFL. (3) Power enhancements of RDFFL to record kilowatt level are demonstrated with the aid of fiber master oscillator power amplifier (MOPA) with different pump schemes.