Femtosecond Fiber Amplifier Research Articles

Gain-induced Kerr beam cleaning in a femtosecond fiber amplifier

Kerr beam cleaning is a nonlinear phenomenon in graded-index multimode fiber where power flows toward the fundamental mode, generating bell-shaped output beams. Here we study beam cleaning of femtosecond pulses accompanied by gain in a multimode fiber amplifier. Mode-resolved energy measurements and numerical simulations showed that the amplifier generates beams with high fundamental mode content (greater than 30% of the overall pulse energy) for a wide range of amplification levels. Control experiments using stretched pulses that evolve without strong Kerr nonlinear effects showed a degrading beam profile, in contrast to nonlinear beam cleaning. Temporal measurements showed that seed pulse parameters have a strong effect on the amplified pulse quality. These results may influence the design of future high-performance fiber lasers and amplifiers.

Two-photon absorption spectrometers for near infrared

We present a Silicon-based Charge-Coupled Device (Si-CCD) sensor applied as a cost-effective spectrometer for femtosecond pulse characterization in the Near Infrared region in two different configurations: two-Fourier and Czerny-Turner setups. To test the spectrometer's performance, a femtosecond Optical Parametric Oscillator with a tuning range between 1100 and 1700nm and a femtosecond Erbium-Doped Fiber Amplifier at 1582nm were employed. The nonlinear spectrometer operation is based on the Two-Photon Absorption effect generated in the Si-CCD sensor. The achieved spectrometer resolution was 0.6 ± 0.1nm with a threshold peak intensity of 2×106Wcm2. An analysis of the nonlinear response as a function of the wavelength, the response saturation, and the criteria to prevent it are also presented.

Performance Enhancements of Femtosecond Fiber Amplifier by Pump Wavelength Optimization

We demonstrate an efficient scheme to accelerate the self-similar pulse evolution and reduce the intensity noise of a free-running femtosecond fiber amplifier based on the pump wavelength optimization. Experiments and simulations indicate the enhanced tolerances of the pulse self-similar amplification to the seed signal power and pump wavelength fluctuations, with the optimum 915 nm pump wavelength. ∼20% increase in the compressed pulse quality and ∼31% reduction in the amplifier root-mean-square (RMS) relative intensity noise (RIN) (1.5 kHz to 5 MHz) have been observed, even with a more than 4 times higher pump laser diode (LD) RIN than the case of 976 nm. ∼50 fs transform-limited pulses are generated with the ∼0.03% amplifier RMS RIN. The proposed scheme can lower the requirements of low-noise self-similar femtosecond fiber amplifiers on the power stability of the seed oscillator and the thermal control of the pump LD, thus denoting potentials for the various satellite-based high-precision applications of femtosecond laser in space.

Orbital angular momentum beams generation from 61 channels coherent beam combining femtosecond digital laser.

We report on the use of a 61 beamlets coherent beam combination femtosecond fiber amplifiers as a digital laser source to generate high-power orbital angular momentum beams. Such an approach opens the path for higher-order non-symmetrical user-defined far field distributions.

Highly coherent multi-octave polarization-maintained supercontinuum generation solely based on ZBLAN fibers.

We present a highly stable polarization-maintained supercontinuum (SC) using a setup solely based on ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fibers. The pumping source consists of a femtosecond master oscillator fiber amplifier based on thulium-doped ZBLAN fibers. It provides multi-watts of output power with the center wavelength of 1920 nm at 1 MHz repetition rate. The SC generated by pumping an elliptical core passive single-mode ZBLAN fiber spans from 350 nm to 4.5 µm and exhibits high stability. We characterized the SC pulse using sum-frequency cross-correlation frequency resolved optical gating.

Spectral compression in a multipass cell.

Starting from a femtosecond ytterbium-doped fiber amplifier, we demonstrate the generation of near Fourier transform-limited high peak power picosecond pulses through spectral compression in a nonlinear solid-state-based multipass cell. Input 260 fs pulses negatively chirped to 2.4 ps are spectrally compressed from 6 nm down to 1.1 nm, with an output energy of 13.5 µJ and near transform-limited pulses of 2.1 ps. A pulse shaper included in the femtosecond source provides some control over the output spectral shape, in particular its symmetry. The spatial quality and spatio-spectral homogeneity are conserved in this process. These results show that the use of multipass cells allows energy scaling of spectral compression setups while maintaining the spatial properties of the laser beam.

Coherent beam combining of seven fiber chirped-pulse amplifiers using an interferometric phase measurement.

Coherent beam combining in tiled-aperture configuration is demonstrated on seven femtosecond fiber amplifiers using an interferometric phase measurement technique. The residual phase error between two fibers is as low as λ/55 RMS and a combination efficiency of 48% has been achieved. The combined pulses are compressed to 216 fs, delivering 71 W average power at a repetition rate of 55 MHz. Operating the laser system in a nonlinear regime with an estimated B-integral of 5 rad yields a combining efficiency of 45% with the same phase stability. These results pave the way to very large high-power and high energy coherent beam combining systems.

Thulium-doped nonlinear fiber amplifier delivering 50 fs pulses at 20 W of average power.

In this Letter, we present an optimized nonlinear amplification scheme in the 2μm wavelength region. This laser source delivers 50fs pulses at an 80MHz repetition rate with exceptional temporal pulse quality and 20W of average output power. According to predictions from numerical simulations, it is experimentally confirmed that dispersion management is crucial to prevent the growth of side pulses and an increase of the energy content in a temporal pedestal surrounding the self-compressed pulse. Based on these results, we discuss guidelines to ensure high temporal pulse quality from nonlinear femtosecond fiber amplifiers in the anomalous dispersion regime.

Spatio-spectral structures in high harmonic generation driven by tightly focused high repetition rate lasers

We investigate the spatio-spectral properties of extreme ultraviolet (XUV) high harmonic radiation driven by high repetition rate femtosecond laser systems. In the spatio-spectral domain, ring-shaped structures at each harmonic order associated with long-trajectory electrons are found to form arrow-shaped structures at the cutoff. These structures are observed with two different laser systems: an optical parametric chirped-pulse amplifier system at a central wavelength of 1.55 μm and 125 kHz repetition rate, and a temporally compressed femtosecond ytterbium fiber amplifier at 1.03 μm wavelength and 100 kHz repetition rate. As recently pointed out, the observed structures are well explained by considering the space–time atomic dipole-induced phase for short and long electron trajectories in the generation plane. The tighter focusing geometry and longer wavelength associated with these emerging driving laser systems increase the ring-like divergence and spectral broadening for high harmonics. Cutoff energies and photon fluxes obtained in argon and neon are also reported. Overall, these results shed new light on the properties of XUV radiation driven by these recently developed high average power laser systems, paving the way to high photon-flux XUV beamlines.

High-energy few-cycle Yb-doped fiber amplifier source based on a single nonlinear compression stage.

A simple, compact, and efficient few-cycle laser source at a central wavelength of 1 µm is presented. The system is based on a high-energy femtosecond ytterbium-doped fiber amplifier delivering 130 fs, 250 µJ pulses at 200 kHz, corresponding to 1.5 GW of peak power and an average power of 50 W. The unprecedented short pulse duration at the output of this system is obtained by use of spectral intensity and phase shaping, allowing for both gain narrowing mitigation and the compensation of the nonlinear accumulated spectral phase. This laser source is followed by a single-stage of nonlinear compression in a xenon-filled capillary, allowing for the generation of 14 fs, 120 µJ pulses at 200 kHz resulting in 24 W of average power. High-harmonic generation driven by this type of source will trigger numerous new applications in the XUV range and attosecond science.

Limits of femtosecond fiber amplification by parabolic pre-shaping

We explore parabolic pre-shaping as a means of generating and amplifying ultrashort pulses. We develop a theoretical framework for modeling the technique and use its conclusions to design a femtosecond fiber amplifier. Starting from 9 ps pulses, we obtain 4.3 μJ, nearly transform-limited pulses 275 fs in duration, simultaneously achieving over 40 dB gain and 33-fold compression. Finally, we show that this amplification scheme is limited by Raman scattering, and outline a method by which the pulse duration and energy may be further improved and tailored for a given application.

Supercontinuum-seeded few-cycle mid-infrared OPCPA system.

We propose and demonstrate an OPCPA architecture emitting few-cycle pulses at 3070 nm and 1550 nm based on a high-energy femtosecond ytterbium-doped fiber amplifier pump. The short pump pulse duration allows direct seeding by a supercontinuum in the 1.4 - 1.7 µm signal range, generated in bulk YAG. It also allows a simplified dispersion management along the system and broad optical gain bandwidth. The dual output system delivers 20 µJ, 49 fs signal pulses at 1550 nm and 10 µJ, 72 fs idler pulses at 3070 nm. Power scaling limitations due to beam distortion in the last MgO:PPLN-based OPCPA stage are discussed and investigated.

On the Efficiency of Parabolic Self-Similar Pulse Evolution in Fiber Amplifiers with Gain Shaping

We numerically and experimentally demonstrate the influences of gain fiber length, initial pulse center wavelength, duration, chirp, and temporal profile for efficient parabolic self-similar evolution, accounting for the strong gain shaping effect in fiber amplifiers under high gains. A spectrally resolved numerical model allowing for realistic descriptions of the variable wavelength-dependent gain is developed. The results predict the specific regions of the initial center wavelength and the duration for the self-similar evolution, which move toward a longer center wavelength and a broader duration range with the increasing gain fiber length. For short-length high-gain amplifiers, a proper negative initial chirp can not only resist the gain-shaping deformation and facilitate the self-similar evolution but also provide a broadband output. In addition, a triangular initial profile is found to be of minimum sensitivity to the detrimental gain shaping and optimum for efficient self-similar evolution. The experimental results confirm the numerical predictions. The studies of the fast parabolic pulse formation in short-length high-gain amplifiers presented here demonstrate the potential for performance scaling of femtosecond fiber amplifiers, few-cycle pulse sources, high-power frequency combs, and related applications.

Comparison of femtosecond pulse dynamics in a PCF amplifier with backward and forward pump schemes

A comparative study of femtosecond pulse dynamics in a photonic crystal fiber amplifier with backward and forward pump schemes is demonstrated experimentally and numerically. The output power, pulse duration and spectrum width of the amplified soliton pulse at different pump powers under the two pump schemes are discussed. The results show that the evolution of the soliton pulse shape in the temporal and spectral domains is also dependent on the direction of pump besides the output power. A narrower pulse duration and spectral width (i.e. smaller time-bandwidth-product), the much less nonlinearity accumulation (i.e. smaller B-Integral value) and a higher output power can be obtained in the backward pump scheme compared to that in the forward pump scheme. The physical origin can be explained by the interaction among group-velocity dispersion, self-phase modulation and nonuniform gain distribution along the fiber due to different end-pump directions. The results provide convincing support to choose the backward pump scheme rather than the forward pump scheme in soliton amplification region in a femtosecond fiber amplifier.

Spectral and spatial full-bandwidth correlation analysis of bulk-generated supercontinuum in the mid-infrared.

We report the measurement of spectral and spatial correlations in supercontinua generated by focusing microjoule pulses from a femtosecond ytterbium-doped fiber amplifier laser in bulk YAG. The measurement is full-bandwidth at a repetition rate of 1 MHz owing to the use of time-stretch dispersive Fourier transform technique. In contrast with fiber-based supercontinuum generation, our results show an excellent stability of the spectral and spatial properties of the output supercontinuum, with an essentially correlated behavior in the 1.4-1.7 μm wavelength range. These results provide strong ground for the development of supercontinuum-seeded ultrafast optical parametric amplifier systems in the mid-infrared using ytterbium lasers as pump sources.

Er-fiber femtosecond optical frequency comb covering visible light

Femtosecond optical frequency combs (FOFCs) with output wavelengths covering visible light have potential applications in absolute frequency measureflent of iodine-stabilized lasers and optical clock lasers. Based on optical amplification, frequency doubling and spectrum broadening, a home-made Er-fiber femtosecond optical frequency comb (Er-FOFC) with output wavelengths covering visible light is demonstrated. One path with an average power of 8 mW from Er-FOFC is used as the seed pulse for spectrum broadening to cover the visible light. This path is first amplified to 532 mW by injecting into an Er-doped femtosecond fiber amplifier with combined forward and backward pumping and then frequency doubled with a MgO: PPLN crystal with an output power of 85 mW, frequency-doubling efficiency of 32% and pulse duration of 85fs. The output power of this path can be first amplified to 532 mW through an Er-doped femtosecond fiber amplifier when the forward pumping and backward pumping both turn on. Then the frequency-doubling laser can be generated in a MgO: PPLN crystal. The frequency-doubling efficiency is 32% and the pulse duration is 85 fs; the frequency-doubling light is spectrally broadened from 500 to 1000 nm in a photonic crystal fiber (PCF), with an output power of 85 mW and coupling efficiency of 50%. To verify the performance of the broadened spectrum, the light from the Er-FOFC and a compact iodine-stabilized frequency-doubled Nd: YAG laser at 532 nm is beaten. A beat signal with a signal-to-noise ratio of 30 dB at 100 kHz RBW is obtained, which provides a useful tool for absolute frequency measureflent of visible lasers.

Two-channel pulse synthesis to overcome gain narrowing in femtosecond fiber amplifiers

We demonstrate spectral coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using active phase stabilization based on an electro-optic phase modulator, an average power of 10 W before compression and a high gain factor of 30 dB are obtained. At this gain value, 130 fs pulses with a spectral width of 19 nm can be generated, highlighting the strong potential of pulse synthesis for the reduction of the minimum duration of ultrashort pulses in fiber chirped-pulse amplifiers.

Generation of 0.3 mW high-power broadband terahertz pulses from GaP crystal pumped by negatively chirped femtosecond laser pulses

Based on optical rectification in a 3 mm GaP crystal with a femtosecond photonic crystal fiber amplifier as the pump source, a detailed experimental investigation on the influence of the chirp characteristics of the pump pulses on the THz wave generation efficiency is conducted. It is shown that a higher THz generation efficiency can be achieved using negatively chirped pulses rather than de-chirped pulses. In the chosen geometry, using 21 W pump pulses with a negative chirp of −2.3 × 10−3 ps2, broadband THz radiation with an average power of 0.3 mW is obtained, which is, to the best of our knowledge, the highest value reported so far for high-repetition-rate THz pulses generated by collinear optical rectification. The higher THz generation efficiency by negatively chirped pulses is explained by a pulse-narrowing mechanism in the nonlinear crystal.

Femtosecond fiber chirped- and divided-pulse amplification system

We implement both chirped pulse amplification and divided pulse amplification in the same femtosecond fiber amplifier setup. This scheme allows an equivalent stretched pulse duration of 2.4 ns in a compact tabletop system. The generation of 77 W of compressed average power at 4.8 MHz, together with 320 fs and 430 μJ pulses at a repetition rate of 96 kHz, is demonstrated using a distributed mode-filtering, rod-type, ytterbium-doped fiber. Limitations in the temporal recombining efficiency due to gain saturation inside the fiber amplifier are identified.

Optimization of femtosecond Yb-doped fiber amplifiers for high-quality pulse compression

We both theoretically and experimentally investigate the optimization of femtosecond Yb-doped fiber amplifiers (YDFAs) to achieve high-quality, high-power, compressed pulses. Ultrashort pulses amplified inside YDFAs are modeled by the generalized nonlinear Schrödinger equation coupled to the steady-state propagation-rate equations. We use this model to study the dependence of compressed-pulse quality on the YDFA parameters, such as the gain fiber's doping concentration and length, and input pulse pre-chirp, duration, and power. The modeling results confirmed by experiments show that an optimum negative pre-chirp for the input pulse exists to achieve the best compression.