Yb-doped Fiber Amplifier Research Articles

Chair-like pulses in an all-normal dispersion Ytterbium-doped mode-locked fiber laser.

We report, for what we believe is the first time, generation of stable chair-like pulses (a pulse shape with an initial long flat portion followed by a short high peak power portion resembling the shape of a chair) by mode locking of a Ytterbium (Yb)-doped fiber laser. Chair-like pulse shapes are achieved by implementing dual saturable absorbers, one based on a nonlinear optical loop mirror (NOLM) and the other based on nonlinear polarization rotation (NPR) inside the cavity. The transmission characteristics of the NOLM-NPR pair leading to the formation of chair-like pulses are numerically investigated. We also report the amplification characteristics of chair-like pulses in an external multistage Yb-doped fiber amplifier setup at different repetition rates of the pulse train. It was found that the chair-like pulses are suitable for amplification, and more than 10 W of average power at 460 kHz repetition rate have been obtained at total pump power of ∼20 W coupled to the power amplifier. At a lower repetition rate (115 kHz), ∼8 W of average power were obtained corresponding to ∼70 μJ of pulse energy with negligible contribution from amplified spontaneous emission or stimulated Raman scattering. We believe that such an oscillator-amplifier system could serve as an attractive tool for micromachining applications.

Long-term mode shape degradation in large mode area Yb-doped pulsed fiber amplifiers

Experimental observation of long-term output mode shape degradation in pulsed Yb-doped fiber amplifiers is reported for the first time. The process occurs in large mode area Yb-doped fibers and is caused by the formation of the long period grating responsible for power transfer from the fundamental mode to the first high-order mode. The linkage between the process and photodarkening was revealed.

Numerical modeling of the impact of pump wavelength on Yb-doped fiber amplifier performance

Ytterbium-doped optical amplifiers have become common tools for industrial applications due to their high efficiency, relatively low cost and potentially very high output power level. The efficiency of an ytterbium-doped fiber amplifier depends mainly on the absorption of pump radiation, and, therefore, optimum pump wavelengths have been proposed such as 915 nm. However, the semiconductor pump diodes batch supplied by manufacturers may exhibit a spread in the output wavelength. This paper theoretically investigates the performance of Yb-doped amplifiers for different pump wavelengths and defines the pump power penalty when the pump source does not emit at the optimum wavelength. The penalty has been defined as normalized excess pump power required to achieve the desired gain.

Beam combinable, kilowatt, all-fiber amplifier based on phase-modulated laser gain competition.

We report power scaling results of a highly efficient narrow-linewidth monolithic Yb-doped fiber amplifier seeded with two signals, operating at 1038 and 1064 nm. With the appropriate seed power ratio applied, this technique was shown to suppress stimulated Brillouin scattering in conjunction with phase modulation, while generating the output power in predominantly the longer wavelength signal. Notably, the integration of laser gain competition with pseudo-random bit sequence phase modulation, set at a clock rate of 2.5 GHz and utilizing an optimized pattern to match the shortened effective nonlinear length, yielded 1 kW of output power. The beam quality was measured to be near the diffraction limit with no sign of transverse mode instability. Furthermore, the coherent beam combination performance of the amplifier provided a 90% combining efficiency with no indication of spectral broadening when compared to the single-tone case. Overall, the power scaling results represent a significant reduction in spectral linewidth compared to that of commercially available narrow-linewidth Yb-doped fiber amplifiers.

Theoretical and experimental research on the ∼980-nm Yb-doped fiber laser

The output properties of the ∼980-nm Yb-doped fiber laser versus pump power and core–cladding ratio of gain fiber, also the amplified spontaneous emission (ASE) at different wavelengths of seed laser, are investigated theoretically. An all-fiber amplifier based on different wavelengths of seed laser at 974.4, 977, and 981.7 nm brings the studies on parasitic oscillation and ASE in the ∼980-nm Yb-doped fiber amplifier. Through the theoretical and experimental research, we found that the controlling of three-level ASE around ∼980-nm is pivotal for obtaining a high-power 980-nm Yb-doped fiber amplifier.

Stimulated Brillouin Scattering Enhancement Factor Improvement in a 11.6-GHz-Linewidth 1.5-kW Yb-Doped Fiber Amplifier**Supported by the National Natural Science Foundation of China under Grant Nos U1330134, 61308024 and 11174305, the National High-Technology Research and Development Program of China under Grant No 2014AA041901, and the Shanghai Natural Science Foundation under Grant No 11ZR1441400.

The stimulated Brillouin scattering (SBS) threshold enhancement factor in a pure white noise linewidth broadening Yb-doped fiber amplifier (YDFA) with a short large mode area fiber is theoretically and experimentally studied. We demonstrate a 1064.08 nm, 11.6 GHz linewidth, 1.5 kW output power YDFA with an SBS threshold enhancement of ~57 (26 W SBS threshold with single frequency seed). The output beam is near-diffraction limited with a beam quality factor of M2 = 1.15 and a slope efficiency of up to 87%. No SBS or stimulated Raman scattering effects are observed in the whole power range. Further power scaling is limited by the available pump power in our system.

Generation of synchronized picosecond pulses by a 1.06-µm gain-switched laser diode for stimulated Raman scattering microscopy.

We propose that a gain-switched laser diode (GS-LD) can be used as a picosecond laser source for stimulated Raman scattering (SRS) microscopy. We employed a 1.06-µm GS-LD to generate ~13-ps pulses at a repetition rate of 38 MHz and amplified them to >100 mW with Yb-doped fiber amplifiers. The GS-LD was driven by 200-ps electrical pulses, which were triggered through a toggle flip-flop (T-FF) so that the GS-LD pulses were synchronized to Ti:sapphire laser (TSL) pulses at a repetition rate of 76 MHz. We found the timing jitter of GS-LD pulses to be approximately 2.7 ps in a jitter bandwidth of 7 MHz. We also show that the delay of electrical pulses can be less sensitive to the optical power of TSL pulses by controlling the threshold voltage of the T-FF. We demonstrate the SRS imaging of polymer beads and of HeLa cells with GS-LD pulses and TSL pulses, proving that GS-LD is readily applicable to SRS microscopy as a compact and stable pulse source.

Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities.

The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

Bidirectional pumped high power Raman fiber laser.

This paper presents a 3.89 kW 1123 nm Raman all-fiber laser with an overall optical-to-optical efficiency of 70.9%. The system consists of a single-wavelength (1070nm) seed and one-stage bidirectional 976 nm non-wavelength-stabilized laser diodes (LDs) pumped Yb-doped fiber amplifier. The unique part of this system is the application of non-wavelength-stabilized LDs in high power bidirectional pumping configuration fiber amplifier via refractive index valley fiber combiners. This approach not only increases the pump power, but also shortens the length of fiber by avoiding the usage of multi-stage amplifier. Through both theoretical research and experiment, the bidirectional pumping configuration presented in this paper proves to be able to convert 976 nm pump laser to 1070 nm laser via Yb3+ transfer, which is then converted into 1123 nm Raman laser via the first-order Raman effect without the appearance of any higher-order Raman laser.

Experimental Study on All-Fiberized Continuous-Wave Yb-Doped Fiber Amplifier Operating Near 980 nm

The all-fiberized continuous-wave 980-nm amplifier with an Yb-doped double-cladding fiber (YDCF) is experimentally investigated first, to the best of our knowledge. It is revealed that besides the core-to-cladding ratio of YDCF, the filling of the seed light in the active core is also of great importance for the 980-nm signal amplification. By improving the seed light filling in the active core, the 6.8-W 980-nm radiance is obtained from the 60/130 fiber amplifier and the slope efficiency can reach to 32%. Besides, it is also revealed that not only the 1064-nm, but also the 985-nm amplified spontaneous emission will be induced in the YDCF amplifier operating near 980 nm.

Sub-Microsecond Pulsed Pumping as a Means of Suppressing Amplified Spontaneous Emission in Tandem Pumped Fiber Amplifiers

We propose a new tandem pumping scheme in three-level rare-earth-doped optical fibers, where energy storage in a pump fiber laser enables much shorter ( $\sim 500$ ns) pump pulses than is possible with pulsed diode laser pumping. This has the effect of reducing the time during which the final amplifier has high inversion and, hence, reduces the amplified spontaneous emission (ASE) when compared with continuous wave (CW) tandem pumping. We simulate the gain and ASE dynamics in a pulse-pumped Yb-doped fiber amplifier and show that the ratio of amplified signal to ASE background can be improved by up to two orders of magnitude over a large range of pulse energies compared with CW tandem pumping. Sub-microsecond pulsed tandem pumping offers a relatively straightforward, as yet experimentally untried, way to substantially reduce undesirable ASE compared with the levels in diode-pumped fiber amplifiers.

Enhanced Supercontinuum Generation in Nonlinear Ytterbium-Doped Fiber Amplifier by Seeding at Short Wavelength

The amplification of a short-pulsed fiber laser along a high-power nonlinear fiber amplifier will undergo nonlinear spectral broadening and temporal distortion with the influence of fiber nonlinear effects. Within the same nonlinear Yb-doped fiber amplifier, we experimentally compared the different spectral and temporal evolution processes of tunable chirped picosecond pulses at different wavelengths (ranging from 1030 to 1070 nm). The comparison results indicate that the generated supercontinuum can be significantly enhanced when the amplifier is seeded at a short wavelength. A comprehensive analysis links the phenomenon to the overlap of Yb $^{3+}$ ion gain and the Raman gain, as well as the additional amplified spontaneous emission seeding effect. The phenomena of nonlinear-effect-induced spectral and temporal center dips were also observed and explained. The proposed conclusion can help to thoroughly understand the nonlinear process in fiber amplifiers and contribute to the development of a high-power spectrally flat amplifier-based supercontinuum source.

High-power near-infrared linearly-polarized supercontinuum generation in a polarization-maintaining Yb-doped fiber amplifier.

We report an all-fiber linearly-polarized (LP) supercontinuum (SC) source with high average power generated in a polarization-maintaining (PM) master-oscillation power-amplifier (MOPA). The experimental configuration comprises an LP picosecond pulsed laser and three PM Yd-doped fiber amplifiers (YDFA). The output has the average power of 124.8 W with the spectrum covering from 850 to 1900 nm. The measured polarization extinction ratio (PER) of the whole SC source is about 85% which verifies the SC an LP source. This work is, to our best knowledge, the highest output average power of an LP SC source that ever reported. The influence of PM fiber splicing method on the LP SC property is investigated by splicing the PM fibers with slow axis parallel or perpendicularly aligned, and also an LP SC with low output power is demonstrated.

Modeling of coherent beam combining from multimillijoule chirped pulse tapered fiber amplifiers

The amplification of high energy chirped pulses in Large Mode Area tapered fiber amplifiers and their coherent combining have been investigated numerically. We have developed a three-dimensional model of strongly chirped nanosecond pulse amplification and compression back to femtosecond duration fully taking into account transverse and longitudinal variations of refractive index profile and distribution of active ions in the fiber, wavelength dependence of emission and absorption cross sections, gain saturation and Kerr nonlinearity. Modeling of Yb-doped fiber amplifier shows that up to 3 mJ of output energy can be extracted in 1 ns pulse with single-mode beam quality. Finally, we have investigated numerically the capabilities of compression and coherent combining of up to 36 perturbed amplifying channels in which high-order modes were excited and have obtained more than 70% combining efficiency and 380 fs compressed pulse duration.

Generation of 130 W narrow-linewidth high-peak-power picosecond pulses directly from a compact Yb-doped single-stage fiber amplifier

We report a compact, 130-W single-stage master oscillator power amplifier with a high peak power of 51.3 kW and a narrow spectral linewidth of 0.1 nm. The seed source is a single-mode, passively mode-locked solid-state laser at 1064 nm with an average power of 2 W. At a repetition rate of 73.5 MHz, the pulse duration is 30 ps. After amplification, it stretches to 34.5 ps. The experiment enables the optical-to-optical conversion efficiency to reach 75%. To the best of our knowledge, this is the first report of such a high-power, narrow spectral linewidth, high peak power picosecond-pulse fiber amplifier based on a continuous-wave, mode-locked solid-state seeding laser. No amplified spontaneous emission and stimulated Raman scattering were observed when the pump was increased.

Offset-free broadband Yb:fiber optical frequency comb for optical clocks.

We demonstrate a passively offset-frequency stabilized optical frequency comb centered at 1060 nm. The offset-free comb was achieved through difference frequency generation (DFG) between two portions of a supercontinuum based on a Yb:fiber laser. As the DFG comb had only one degree of freedom, repetition frequency, full stabilization was achieved via locking one of the modes to an ultra-stable continuous wave (CW) laser. The DFG comb provided sufficient average power to enable further amplification, using Yb-doped fiber amplifier, and spectral broadening. The spectrum spanned from 690 nm to 1300 nm and the average power was of several hundred mW, which could be ideal for the comparison of optical clocks, such as optical lattice clocks operated with Sr (698 nm) and Hg (1063 nm) reference atoms.

Burst-mode Yb-doped fiber amplifier system optimized for low-repetition-rate operation

We report on the burst-mode operation of a polarization-maintaining Yb-doped multi-stage all-fiber amplifier capable of generating 10-pulse 100 ns long bursts of 400 μJ total energy. The corresponding average energy per pulse is 40 μJ, with a standard deviation of 16%. The 40 μJ pulses are compressible to a full width at half-maximum of 500 fs. The burst repetition rate is set to 1 kHz and the amplifier is synchronously pulse pumped to minimize amplified spontaneous emission (ASE) between the bursts. A special amplifier design has been developed to suppress ASE further at the cost of lower power efficiency. A detailed investigation of gain and ASE suppression performance of the amplifier in relation to pulsed pumping is presented.

Mitigating of modal instabilities in linearly-polarized fiber amplifiers by shifting pump wavelength

We investigated the effect of pump wavelength on the modal instabilities (MI) in high-power linearly polarized Yb-doped fiber amplifiers. We built a novel semi-analytical model to determine the frequency coupling characteristics and power threshold of MI, which indicates promising MI suppression through pumping at an appropriate wavelength. By pumping at 915 nm, the threshold can be enhanced by a factor of 2.1 as compared to that pumped at 976 nm. Based on a high-power linearly polarized fiber amplifier platform, we studied the influence of pump wavelength experimentally. A maximal enhancement factor of 1.9 has been achieved when pumped at 915 nm, which agrees with the theoretical calculation and verified our theoretical model. Furthermore, we show that MI suppression by detuning the pump wavelength is weakened for fiber with a large core-to-cladding ratio.

Nonlinear compression of high energy fiber amplifier pulses in air-filled hypocycloid-core Kagome fiber.

We report on the generation of 34 fs and 50 µJ pulses from a high energy fiber amplifier system with nonlinear compression in an air-filled hypocycloid-core Kagome fiber. The unique properties of such fibers allow bridging the gap between solid core fibers-based and hollow capillary-based post-compression setups, thereby operating with pulse energies obtained with current state-of-the-art fiber systems. The overall transmission of the compression setup is over 70%. Together with Yb-doped fiber amplifier technologies, Kagome fibers therefore appear as a promising tool for efficient generation of pulses with durations below 50 fs, energies ranging from 10 to several hundreds of µJ, and high average powers.

Modeling of the spectral properties of CW Yb-doped fiber amplifier and experimental validation

We present a theoretical model for the spectral properties of continuous wave ytterbium-doped fiber amplifer (YDFA) based on master oscillator power amplifer (MOPA) structure. Using the combined simulation of the power balanced equations and the nonlinear Schrödinger equations, the spectral shape can be calculated in both the oscillator stage and the amplifier stage. The predictions of the theory are in excellent quantitative agreement with the experimental results. This theory provides a useful tool for analyzing the spectral properties of the YDFA based on MOPA structure.