Fiber Master Oscillator Power Amplifier Research Articles

A >20-W, linearly polarized single-frequency continuous-wave all-fiber laser at ∼1540 nm

We have developed both theoretically and experimentally an all polarization-maintaining fiber-based master oscillator power amplifier (MOPA) system, which allows for amplifying the single-frequency continuous-wave (CW) fiber laser to >20 W without compromising the laser intensity noise (−110 dBc/Hz@ relaxation oscillation peak) at the specific wavelength of ∼1540 nm. The experimental results agree well with the simulated results. To the best of our knowledge, this is the first demonstration of a ∼1540-nm single-frequency all fiber laser and reaching such a high-power level. The amplified laser features a narrow linewidth of ∼1.4 kHz and a spectral side-mode-suppression ratio of >38 dB, even as the laser wavelength is tuned from 1538.216 nm to 1540.616 nm. It is believed to be the narrowest laser linewidth realized in the <1550-nm wavelength band of an Erbium-Ytterbium co-doped fiber MOPA system. The all polarization-maintaining configuration of the MOPA ensures the amplified laser linearly polarized with the polarization extinction ratio of >22 dB and power root mean square fluctuations of <0.2 %@1h. This high-performance single-frequency fiber laser is expected to be an ideal pump source in the spin-exchange relaxation-free (SERF)-based atomic sensors after second harmonic generation.

208 W single-frequency 1064 nm laser based on a single-crystal fiber master-oscillator power amplifier.

High-power all-solid-state continuous-wave (CW) single-frequency laser with high linear polarization is a significant source for quantum optics and precision measurement. In this Letter, a high-power linearly polarized CW single-frequency laser based on the single-crystal fiber (SCF) master-oscillator power amplifier (MOPA) is presented, in which a homemade 140 W low-noise CW single-frequency laser and a Nd:YAG SCF are firstly employed as the seed laser and the medium of the MOPA, respectively. The mode-matching between the pump laser propagated with waveguide form and the freely propagated seed laser is optimized by considering the influence of the degradations of the polarization and the beam quality. Finally, when the incident powers of the pump and seed lasers are 262.6 W and 126.3 W, respectively, the seed waist radius is optimized to 200 μm. In this case, the output power of the linearly polarized laser reaches up to 208 W, which is the highest output power, to the best of our knowledge. The presented results provide a good reference for implementing a high power and high degree of the polarization and good beam quality laser based on the SCF MOPA.

High-power, highly efficient, all-fiber wavelength-tunable polarization-maintaining Tm-doped fiber MOPA system

A 2 μm all-fiber wavelength-tunable polarization-maintaining Tm-doped fiber master oscillator power amplifier (MOPA) system is presented. We demonstrate an all-fiber ring cavity Tm-doped fiber master-oscillator (MO) with a tuning range of 60 nm (1959.9–2020.1 nm). The maximum output power of 82.4 mW is obtained at 1991 nm with the launched pump power of 16 W at 793 nm, corresponding to a slope efficiency of 10.8 % concerning launched pump power. Finally, through the first stage amplification, the power is successfully amplified to 38.92 W, the slope efficiency is about 31.4 %, and the polarization extinction ratio (PER) reaches 21.4 dB. Then a theoretical simulation is carried out to calculate the influence of different fiber lengths on the slope efficiency and output power of the amplifier stage when the pump power is increased to 420 W, which provides theoretical guidance for the subsequent experiments. Through the simulation, it is found that the output power will be greatly improved by choosing the appropriate fiber length and increasing the pump power.

Inter-/intra-pulse dual-wavelength-operated mid-infrared optical parametric oscillator.

We demonstrate a pulsed mid-infrared (MIR) optical parametric oscillator (OPO) with both inter-pulse and intra-pulse dual-wavelength operation capability. A fiber master oscillator power amplifier incorporating an acousto-optic tunable filter (AOTF) was employed as the pump for the OPO. By finely adjusting the drive wave packets for the AOTF, dual-wavelength pump can be realized within each pulse or between two adjacent pulses. These special temporal-spectral behaviors of the pump can be transferred to MIR via an OPO. In the proof-of-principle experiments, two pump wavelengths at ∼1065 and ∼1076 nm were generated and amplified to ∼31.2 W with equivalent spectral intensities for both pulsation modes. At the highest pump power, total idler power of ∼3.5 W was achieved at ∼3.45 and ∼3.55 µm under both pulsation modes. To the best of our knowledge, this is the first demonstration of both inter-pulse and intra-pulse dual-wavelength MIR generation via an OPO with an identical configuration. It is believed that our design may provide a promising solution to many practical applications including differential absorption lidar and tunable terahertz wave generation.

Gain-switched 3 μm dysprosium-doped fluoride fiber laser pumped at 1.7 μm

To the best of our knowledge, we demonstrated a gain-switched 3 μm dysprosium-doped fluoride fiber laser pumped by a 1706.5 nm pulsed thulium-doped fiber master oscillator power amplifier for the first time. The maximum average power of the 3 μm pulsed laser was 50 mW with a slope efficiency of 12.3%, a repetition rate of 100 kHz, and a pulse width of 283 ns. This work exhibits the potential of 1.7 μm pulse pumped dysprosium-doped fluoride fiber laser as a platform for developing pulsed sources in the 3 μm region.

Kerr-lens mode-locked 49-fs Tm3+:YScO3 single-crystal laser at 2.1 µm.

We report on a continuous wave (CW) and Kerr-lens mode-locked (KLM) Tm3+:YScO3 single-crystal laser centered at 2.1 µm. Efficient CW laser operation with a maximum slope efficiency of 51% was achieved under in-band pumping by an Er:Yb fiber master oscillator power amplifier (MOPA). In KLM operation, pulses as short as 49 fs corresponding to seven optical cycles were achieved at a repetition rate of 96.7 MHz with an average output power of 126 mW. Such short pulse durations are enabled by the inhomogeneously broadened emission spectrum of Tm3+:YScO3 extending to above 2200 nm.

Wavelength and repetition rate tunable high peak power dissipative soliton resonance in an all polarization maintaining Yb-doped fiber laser

High peak power dissipative soliton resonance (DSR) with tunable wavelength and flexible repetition rate is experimentally achieved based on an all polarization maintaining master oscillator power amplifier fiber laser. The figure-of-eight oscillator outputs DSR pulse at a fundamental repetition rate of 3.6 MHz with square-shaped temporal pulse duration of 600 ps. By adjusting the intracavity fiber filter, the center wavelength of the DSR pulse can be continuously changed from 1017 to 1042 nm. These wavelength and temporal duration tunable pulses are then coupled into an all-fiber pulse picker to gain another tuning freedom of repetition rate from 100 kHz to 3.6 MHz. Seeded by the flexible pulse, an two-stage all-fiber amplifier is used to boost the average power to 20 W with the polarization extinction ratio of 18.2 dB. After compression, a record pulse peak power of 1.91 MW is achieved at the repetition rate of 100 kHz with pulse duration of 55 ps.

Mitigation of stimulated Raman scattering in a high-power fiber master oscillator power amplifier laser based on a dual-structure fiber grating

Abstract With the increasing power of fiber lasers, single chirped and tilted fiber Bragg gratings (CTFBGs) cannot completely mitigate continuously enhanced system-excited stimulated Raman scattering (SRS). Although improving the loss rate of a single CTFBG or cascading multiple CTFBGs can provide better suppression of the stronger SRS, excessive insertion loss may cause significant attenuation of the output power. Confronting the challenge, we firstly present an SRS mitigation method based on a dual-structure fiber grating in this paper. The dual-structure fiber grating comprises a CTFBG and a fiber Bragg grating structure, which were designed and fabricated on a passive 25/400 double-clad fiber. To evaluate the performance of the grating, a 3 kW fiber master oscillator power amplifier laser is established. The experimental results demonstrate that the SRS mitigation rate of the grating is greater than 30 dB (99.9%), whereas the insertion loss is only approximately 3%, thus allowing for minimal deterioration of the output power. This solves the contradiction between high suppression rate and high insertion loss faced by CTFBGs, which in turn makes dual-structure fiber gratings particularly suitable for mitigating SRS in 3–5 kW high-power fiber lasers.

High brightness in-band pumped fiber MOPA with output power scaling to >4.6k W.

An in-band pumping technique that employs low numerical aperture (NA) and high brightness fiber lasers emitting at 1018nm enables outstanding power scaling performance by increasing pump power handling capacity of fiber components, reducing thermal load and quantum defect. Here, we present design criteria for an in-band pumped fiber master oscillator power amplifier (MOPA) structure, pumped with 1018nm fiber lasers, as well as mitigation strategies for nonlinear effects for scalable single-mode laser operation at multi-kilowatt power levels. In addition, we report experimental demonstration of an in-band pumped MOPA with an output power scaling up to 4.63kW and a slope efficiency of ∼88%. The MOPA system is pumped by 18×264W high brightness fiber lasers operating at 1017.8±0.3n m with an NA of <0.075. The laser emits at 1080.4nm wavelength with a stimulated Raman scattering suppression of >36.8d B and has a near-diffraction-limited beam quality of M 2∼1.61. To the best of our knowledge, our laser has the highest brightness of 153G W⋅c m -2⋅s r -1 reported at 1080nm in co-pumping configuration.

Single mode laser diode based compact seed source for continuous wave Yb-Raman fiber amplifier with enhanced higher order Raman suppression

In this paper, we demonstrated a novel seed source for the continuous wave Yb-Raman fiber amplifier, which adopted an 1140 nm single mode laser diode (LD) as Raman signal seed and a conventional 1085 nm fiber oscillator as the Raman pump seed. By splicing a section of long passive fiber in the Raman pump seed cavity, optimizing the Raman pump seed power and the cooling condition, the higher order Raman threshold was doubled and over 500 W output Yb-Raman fiber amplifier was achieved. In addition, our experiment also proved that, for the first time, splicing long passive fiber in the seed oscillator cavity improved the temporal performance of the first order Raman in the conventional fiber master oscillator power amplifier, which could increase the higher order Raman threshold as a result. The seed structure based on a single mode LD and a long cavity length fiber oscillator, could provide a highly compact and cost effective seed source for the high power Yb-Raman fiber amplifier with enhanced higher order Raman suppression.

All-Fiber Wavelength-Tunable Narrow-Linewidth Polarization-Maintaining Tm-Doped Fiber MOPA System

An all-fiber wavelength-tunable narrow-linewidth polarization-maintaining (PM) Tm-doped fiber master oscillator power amplifier (MOPA) system is presented. We demonstrate an all-fiber ring cavity Tm-doped fiber master-oscillator (MO) with a tuning range of 110 nm (from 1925 to 2034 nm). The maximum output power of 459 mW is obtained at 1992.9 nm for 16 W of launched pump power at 793 nm, corresponding to a slope efficiency of 5.6% concerning launched pump power. By using a one-stage Tm-doped fiber amplifier combined with a high gain of &gt;10 dB, the maximum slope efficiency is 12.6% and the output power is 5.65 W at 1993 nm. The 3 dB linewidth is less than 0.5 nm, M2 ≈ 1.25, and the polarization extinction ratio (PER) reaches 21.4 dB. The influence of different active fiber lengths on laser amplification is also studied.

Broadband, Continuous-Wave, Mid-Infrared Generation Based on ASE Fiber Source

The research reports broadband, continuous-wave (CW), mid-infrared generation based on an optical parametric oscillator (OPO), pumped by an amplified spontaneous emission (ASE) fiber source. The ASE fiber source has a ytterbium (Yb)-doped fiber master oscillator power amplifier (MOPA) structure. By injecting another fiber laser with high power and a narrow linewidth into the nonlinear crystal, the phase-matching difference frequency generation (DFG) can be realized at a low pumping threshold. It is shown that during the down-conversion of the ASE fiber source, the intracavity DFG can steadily and effectively generate a broadband mid-infrared laser, with a central wavelength of 3713 nm. The bandwidth of the mid-infrared laser is 122 nm. The pump power of 2 W generates a 0.238 W mid-infrared laser with a pump-idler conversion efficiency of 11.6%. It has tremendous application potential in the research of broadband CW mid-infrared lasers.

9.14-Mbps 64-PPM UWOC system based on a directly modulated MOPA with pre-pulse shaping and a high-sensitivity PMT with analog demodulation.

A pulsed fiber master oscillator power amplifier (MOPA), which is combined with second harmonic generation (SHG) and modulated by directly changing the current of the low-power seed laser, is designed in this paper to overcome the 'green gap' of semiconductor lasers and the difficulty of obtaining high-power and wide-bandwidth driving circuits. To decrease the guard slot and increase the data rate of a high-order pulse position modulation (PPM) system, pre-pulse shaping (PPS) is utilized to decrease the fluctuation of pulse power, which is caused by the gain dynamics of multi-order amplification of the MOPA, from 55.6% to 27.5% for 25-ns pulses and from 22.4% to 16.7% for 10-ns pulses, respectively. Moreover, an analog PPM demodulation method is proposed to mitigate the nonlinear effect caused by space charge limitations at dynodes of a photomultiplier tube (PMT) and increase the robustness of the system. In an optical darkroom, a 99-m 64-PPM UWOC transmission, of which the measured link loss is around 13.16 attenuation length (AL), is realized in a water tank with a data rate of 9.14 Mbps. The average received optical power ranges from -60.87 to -52.51 dBm, corresponding to a bit error rate (BER) range of 1.93 × 10-4 to 2.3 × 10-3. To further prove the reliability of the proposed system, we implement a 65-m UWOC experiment with the same data rate at a BER of 3.42 × 10-4 in a 50-m standard swimming pool. The maximum link loss is measured to be 15 AL.

High-power ytterbium-doped fibre master-oscillator power-amplifier at 1018 nm

A high-power master-oscillator power-amplifier (MOPA) at 1018 nm employing ytterbium (Yb)-doped fibres as a gain medium is reported. Utilizing a diffraction grating as a reflector, we could successfully suppress the influence of the broadband amplified spontaneous emission on the master-oscillator or the power-amplifier, resulting in stable amplification of the laser signal at 1018 nm. Based on a simple theoretical simulation on gain spectra and experimental investigation on parasitic lasing thresholds, the Yb fibre MOPA constructed in-house yielded 220 W of output at 1018 nm with a beam propagation factor (M2) of 1.1. The prospects for further power scaling are considered.

Gain-switched watt-level thulium-doped fiber laser and amplifier operating at 1.7 μm

Abstract A 1.7 μm gain-switched thulium-doped all-fiber laser with a master oscillator power amplifier (MOPA) configuration, utilizing a bandpass fiber filter and a 1550 nm erbium/ytterbium-codoped fiber MOPA, is demonstrated. The influences of pump pulse parameters (repetition rate and pulse duration) and laser cavity structures (ring and linear) on the laser performances were experimentally investigated. To the best of our knowledge, the power quenching and drop were observed in the 1.7 μm gain-switched thulium-doped fiber lasers for the first time, resulting from the mode-locked-resembling operation and nonlinear effects. Moreover, the fiber ring-cavity laser was more stable than the linear-cavity laser in the time domain and power. Finally, a laser with a maximum average power of 1.687 W, a slope efficiency of 19.7%, a single-pulse energy of 16.87 μJ, a pulse width of 425 ns, a repetition rate of 100 kHz and a peak power of 39.69 W was obtained.

High-power, high-efficiency, all-fiberized-laser-pumped, 260-nm, deep-UV laser for bacterial deactivation

We report a 5.8-W deep-ultraviolet (DUV) laser obtained from frequency-quadrupling of an all-fiberized ytterbium-doped fiber (YDF) master oscillator power amplifier (MOPA). The MOPA system delivers 585 ps pulses at 1040 nm with a maximum available output power of 23.5 W for nonlinear frequency conversion. A lithium triborate (LBO) crystal and a beta barium borate (BBO) crystal are employed for second- and fourth-harmonic generation (FHG), respectively. At a repetition rate of 1.6 MHz, a maximum DUV output power of 5.8 W is obtained at 260 nm with a corresponding pulse energy of 3.6 μJ and maximum peak power of at least 6.9 kW. A 1μm-to-260nm conversion efficiency of 26.4% is achieved at a DUV output power of 5.8 W. To the best of our knowledge these results represent the highest-average-power fiberized-laser-pumped DUV laser, as well as the most efficient DUV generation based on BBO crystals to date. We further demonstrate application of the pulsed DUV laser in bacterial disinfection achieving an inactivation efficiency of 99.999% for E-coli bacteria at a DUV exposure of 7 mJ/cm2.

Compact Single-Frequency MOPA Using a Silica Fiber Highly Doped with Yb3+

We report on a single-frequency fiber master oscillator power amplifier utilizing a polarization-maintaining step-index fiber with an Al/Ce/F core-glass composition doped with a very high Yb concentration (0.25 at.%). This design made it possible to use a very short fiber (~1 m) and to coil it in a tight radius (4 cm in the amplifier, while 2 cm gave similarly negligible bending loss) so that the packaged system is one of the most compact reported to date (~0.6 L). The use of a short fiber increased the threshold for stimulated Brillouin scattering well above 100 W while maintaining near-ideal beam quality. The fiber was pumped with a diode-pumped solid-state laser and cooled passively by spooling it on a grooved aluminum mandrel. The amplifier produced a strongly linearly polarized output at 1064 nm in the fundamental mode (M2 ≤ 1.2) with a 150 kHz linewidth and a power of 81.5 W for 107 W of launched pump power. No deleterious effects from the elevated thermal load were observed. The residual photodarkening loss resulting from the high Yb concentration, found to be small (~0.7 dB/m inferred at 1064 nm) with accelerated aging, reduced the output power by only ~20% after 150 h of operation.

Micromachining of glassy carbon using a Yb-based master oscillator power amplifier nanosecond fiber laser.

Laser machining of glassy carbon was experimentally investigated using a 1065 nm wavelength Yb-based master oscillator power amplifier fiber laser source. Ablation characteristics were determined for SIGRADUR G glassy carbon samples using laser fluence of less than 3J/cm2. The ablation threshold was found to be 0.31J/cm2 for incident pulse repetition rates of 250 and 500 kHz. Glassy carbon exhibited excellent machining characteristics with features of clearly defined edges and smooth surfaces. The ablation rate varied linearly with fluence over the range 0.31-1.3J/cm2. High-quality glassy carbon microstructures were created with optimum laser parameters.

3.5 kW narrow-linewidth monolithic fiber amplifier at 1064 nm by employing a confined doping fiber

A high-power monolithic continuous-wave fiber amplifier at 1064 nm has been demonstrated based on a piece of confined ytterbium-doped fiber, which generated 3.5 kW narrow-linewidth laser with near-diffraction-limited beam quality. By tailoring the diameter of gain dopant across the fiber core, and optimizing the profile of refractive index and doping concentration, the confined doping fiber works making the fundamental mode more dominant in transverse mode competition, while suppressing the high-order mode. In a kilowatt-level narrow-linewidth fiber master oscillator power amplifier (MOPA) system, the threshold of mode instability (MI) is about 2.2 times as much as that of the conventional fiber amplifier. Then, by increasing seed power and utilizing a MOPA configuration with four amplifier stages, up to 3.57 kW signal output, a slope efficiency of 61% without any sign of MI or stimulated Raman scattering for a spectral dynamic range of higher than − 50 d B is obtained. A further increase of the output power was only limited by the available laser-diode pump power. To the best of our knowledge, this is the highest narrow-linewidth laser power generated from the confined doping monolithic fiber amplifier.

High-power 813-nm Tm3+-doped ZBLAN fiber MOPA with signal-light photodarkening suppression.

The photodarkening suppression effect of an 813-nm Tm3+-doped ZBLAN fiber amplifier has been investigated. We have experimentally observed that a photodarkened fiber can be bleached by an 813-nm light and that the photoinduced loss during the amplifier operation is effectively suppressed with the help of the high-power signal. To the best of our knowledge, this is the first investigation of the photodarkening suppression by using a higher power signal in Tm3+-doped fiber amplifiers. Based on these signal photodarkening suppression effects, we have designed a multistage fiber amplifier, and demonstrated the stable operation of the 1.15-W fiber master oscillator power amplifier at 813 nm without additional photobleaching light.