Femtosecond Fiber Laser System Research Articles

Watt-level ultra-flat wideband supercontinuum generation in GeO2 fiber by femtosecond pulse

The high-power ultra-flat supercontinuum (SC) sources play an important role in various fields. In this letter, we experimentally demonstrated an all-fiber, high-power, ultra-flat SC source in GeO2 highly nonlinear fiber (HNLF) pumped by a femtosecond fiber laser system. The system is mode-locked by a semiconductor saturable absorber mirror with an all-polarization-maintaining (PM) structure. Based on the technology of chirped pulse amplification, a maximum power of 3.12 W was realized with a repetition rate of 80 MHz at 1.56 μm, and the pulse duration was compressed to 124 fs in a segment of PM single-mode fiber. Furthermore, by optimizing the length of the HNLF, a wideband SC spectrum of nearly 700 nm was obtained at10 dB with a maximum power of 1.82 W, and the flatness of the spectrum was better than 5 dB in the range of 1600–2200 nm. We observed that the pulse evolved into a noise-like pulse with a narrowest pulse duration of 62 fs. Thanks to the all-PM structure of the fiber laser, the long-term stability of SC power was manifested with a relative fluctuation as low as 0.53 % over 6 h.

Sub-50 fs pulses generation from an all-fiber monolithic gain-managed nonlinear amplifier with CFBG-based pre-compressor

We demonstrate an all-fiber monolithic laser source from a gain-managed nonlinear (GMN) fiber amplifier with a CFBG-based pre-compressor that generates high-contrast pulses with a repetition rate of 80 MHz, average power of 1.6 W, and pulse duration of sub-50 fs. The compressed pulse quality can be optimized by tuning the parameters of the seed pulses injected into the GMN amplifier by controlling the pump power of the fiber pre-amplifier. This compact and cost-effective laser system is a simplified design of the GMN amplification-based femtosecond fiber laser systems. With alignment-free characteristics in the amplifiers and clean compressed pulses, it is easy to assemble and use in applications such as multi-photon microscopy.

All-polarization-maintaining mode-locked thulium-doped femtosecond laser at 1.7 µm

We demonstrate a 1.7 µm femtosecond Tm-doped fiber laser system featuring an all-polarization-maintaining architecture. The seed oscillator is mode-locked by carbon nanotubes and delivers stable pulse centered at 1787.6 nm. With two backward pumped amplifiers, the average power of the laser is amplified to ∼458 mW. Employing proper dispersion management in an all-fiber chirped pulse amplification scheme and the soliton compression effect, we obtained a femtosecond pulse of 206 fs with a pulse energy of 8.8 nJ at a repetition rate of ∼52 MHz. To the best of our knowledge, this is the first demonstration of 1.7 µm femtosecond laser based on a thulium-doped oscillator with all-polarization-maintaining architecture.

In vivo three- and four-photon fluorescence microscopy using a 1.8 µm femtosecond fiber laser system.

Multiphoton microscopy has enabled us to image cellular dynamics in vivo. However, the excitation wavelength for imaging with commercially available lasers is mostly limited between 0.65-1.04 µm. Here we develop a femtosecond fiber laser system that produces ∼150 fs pulses at 1.8 µm. Our system starts from an erbium-doped silica fiber laser, and its wavelength is converted to 1.8 µm using a Raman shift fiber. The 1.8 µm pulses are amplified with a two-stage Tm:ZBLAN fiber amplifier. The final pulse energy is ∼1 µJ, sufficient for in vivo imaging. We successfully observe TurboFP635-expressing cortical neurons at a depth of 0.7 mm from the brain surface by three-photon excitation and Clover-expressing astrocytes at a depth of 0.15 mm by four-photon excitation.

High-power, 474 fs, 31.12 MHz, all-polarization-maintaining, 1560 nm fiber laser system based on the chirped-pulse amplification

We report on a 13.2 W, 474 fs, 31.12 MHz, all-polarization-maintaining (PM), 1560 nm fiber laser system based on the chirped-pulse amplification (CPA). The home-built seed laser with the nonlinear amplified loop mirror (NALM) can deliver two trains of laser pulses. Two stages of cascaded all-PM Er:Yb co-doped fiber (EYDF) amplifiers were employed in scaling up the average power. The corresponding numerical simulation and experimental investigations were implemented in optimizing the power amplification process. To our knowledge, this is the highest output power ever reported for the 1560 nm femtosecond all-PM fiber laser systems.

Compact, repetition rate locked all-PM fiber femtosecond laser system based on low noise figure-9 Er:fiber laser

We demonstrate a compact femtosecond fiber laser system based on all polarization-maintaining (PM) fiber and fiber components integrated structure. The figure-9 oscillator which incorporated a nonlinear amplifying loop mirror in the cavity features a 103.4-MHz high repetition rate with up to 93.1 dB signal-to-noise ratio of the radio frequency spectrum, 0.0056% [1 Hz, 1 MHz] integrated root-mean-square amplitude noise at the fundamental repetition rate and 63.7-fs timing jitter [100 Hz, 1 MHz]. Meanwhile, the fundamental repetition frequency was also locked to a stable radio frequency reference by using a self-designed frequency actuator and a relative frequency stability of 2.1 × 10−12 at 1-s gate time was obtained. Moreover, benefitting from the large positive group-velocity dispersion and negative third-order dispersion at 1.5-μm wavelength band, we also achieved 48.2 fs compressed pulse duration as well as an amplified average power of 199 mW via one-stage all-PM fiber amplifier and compressor. At last, as a performance proof, by directly splicing 38-cm long PM highly nonlinear fiber to the pulse compressor, a broadband coherent supercontinuum spanning from 950 nm to 2150 nm was generated. Our all-PM fiber laser system is suitable for the further buildup of a low noise PM fiber optical frequency comb.

The route to a 200 MHz, all-PM femtosecond Yb-doped fiber laser with a high output coupling ratio.

Based on the time-independent rate equations and nonlinear Schrödinger equation, we simulate a 200 MHz all-polarization-maintaining (PM) mode-locked Yb-doped fiber laser. The cavity round trip evolution toward stable mode locking is present. Additionally, the gain coefficients along the gain fiber as well as the pulses, chirp, and spectra at different locations in the cavity are examined. The effects of chirped fiber Bragg grating parameters on the pulse shape and spectrum profile are also investigated. According to the calculations, we experimentally realize a 200 MHz femtosecond fiber laser with 115 mW output power. The timing jitter and integrated relative intensity noise are measured as 158 fs (1 kHz to 10 MHz) and 0.0513% (1 Hz to 300 kHz), respectively. Eventually, an amplified average power of 610 mW and 79 fs compressed pulses with a peak power of approximately 28 kW are obtained. The exhibited all-PM femtosecond fiber laser system can be adopted as the foundation for an optical frequency comb.

Watt-level gigahertz femtosecond fiber laser system at 920 nm.

We demonstrate a watt-level femtosecond fiber laser system at 0.9 µm with a repetition rate of >1 GHz, which is the highest value reported so far for a fundamental mode-locked fiber laser. The fiber laser system is seeded by a fundamental mode-locked fiber laser constructed with a home-made highly Nd3+-doped fiber. After external amplification and pulse compression, an output power of 1.75 W and a pulse duration of 309 fs are obtained. This compact fiber laser system is expected to be a promising laser source for biological applications, particularly two-photon excitation microscopy.

High power chirped pulse Yb fiber amplifier seeded by mode-locked fiber laser oscillator with multimode interference based saturable absorber

In this research, we demonstrate a novel femtosecond fiber laser system which comprises of ytterbium (Yb) doped mode-locked fiber laser oscillator using nonlinear multimode interference (NL-MMI) based saturable absorber (SA) seeded into a chirped pulse fiber amplifier. The laser pulses from the oscillator centered at a wavelength of 1033.2 nm with a pulse duration of 2.4 ps. Correspondingly, the pulse energy is 120 pJ at the fundamental repetition rate of 21.35 MHz. By integrating the laser oscillator with the amplification system, an output average power of 11 W with compressed pulse duration of 340 fs and pulse energy of 0.51 μJ is achieved. The pulse possesses high stability with signal-to-noise ratio up to 58 dB. These characteristics evince that the fiber amplifier system seeded with NL-MMI based mode-locked laser oscillator results in a highly stable femtosecond laser.

Femtosecond Er-doped fiber laser source tunable from 872 to 1075 nm for two-photon vision studies in humans.

We report the development of a widely-tunable femtosecond fiber laser system and its application for two-photon vision studies. The source is based on an Er-doped fiber laser with spectral shift up to 2150 nm, followed by a second harmonic generation module to generate a frequency-doubled beam tunable from 872 to 1075 nm. The source delivers sub-230 fs pulses with nearly-constant duration over the entire tuning range, with output powers between 0.68-1.24 mW, which corresponds to a pulse energy of 13.2-24.1 pJ. Such pulse energy is sufficient for employing a system for measurements of two-photon scotopic spectral sensitivity of two-photon vision in humans. The laser parameters allow for very efficient and safe two-photon stimulation of the human visual system, as proved by a good separation between one- and two-photon thresholds for wavelengths below 950 nm, which we have confirmed for 3 healthy subjects.

Optical parametric generation in OP-GaAs waveguides pumped by a femtosecond fluoride fiber laser.

We report on mid-infrared optical parametric generation in the 4-5 μm and 9-12 μm bands by pumping custom-designed orientation-patterned gallium arsenide (OP-GaAs) rib waveguides with an ultrafast femtosecond fiber laser system. This pump source is seeded by a mode-locked fluoride fiber laser with 59 MHz repetition rate and can be tuned between 2.8 and 3.2 μm using a soliton self-frequency shifting stage. The single TE and TM modes OP-GaAs crystals feature quasi-phase-matched grating periods of 85 and 90 μm and different transverse sizes thus allowing a wide spectral tunability.

Broadband Mid-IR Light Sources From Difference Frequency Generators Based on a 2-mm-Long Aperiodically-Poled Lithium-Niobate Crystal

We present a difference frequency generator (DFG) based on a 2-mm-long MgO-doped aperiodically-poled lithium-niobate (APPLN) crystal with a linearly-varying poling-period. The DFG was pumped by a femtosecond fiber-laser system with a repetition-rate of 53.4 MHz. It produced broadband mid-infrared light source having an instantaneous-bandwidth covering 1.9–4.6 μm, with an average power of 30 mW. Moreover, by numerical simulations, we revealed that due to the relatively high intensity of the incident signal wave, an APPLN crystal with a length of a few milli-meters was sufficient to produce efficient and broadband nonlinear frequency conversion in a high-repetition-rate, ultra-short-pulsed DFG system.

Recent progress of coherent combining technology in femtosecond fiber lasers

Widely employed in fundamental research, industrial processing, and biomedicine, femtosecond fiber lasers exhibit many attractive features such as high average power, good heat dissipation, excellent beam quality, and compact footprint. Coherent combining technology can effectively suppress the detrimental nonlinear and thermal effects in the fiber amplifiers, and therefore further increase the output pulse energy and average power of femtosecond fiber lasers. In this article, we mainly discuss different coherent combining techniques in high-power ultrafast Yb-fiber laser systems and the relevant phase-locking methods. We believe that the advent of new coherent combining techniques will further improve the average power and pulse energy of femtosecond fiber laser systems, thereby opening up some new research areas.

Development of a Femtosecond High-Power Ytterbium-doped Fiber Laser System and Study on Spiky Spectral Modulation in Highly Nonlinear Chirped-Pulse Fiber Amplifiers

We have developed and analyzed a femtosecond high-power fiber laser system based on chirped-pulse amplification. The system consists of a homemade femtosecond ytterbium-doped fiber oscillator, a fiber pulse stretcher, a pulse picker, three ytterbium-doped fiber amplifiers and a grating-based pulse compressor. A single-mode photonic crystal fiber with a large mode-field diameter of 30 µm is used in the final amplifier, and the special homemade mode-field adapter based on a graded refractive index fiber has been developed for the photonic crystal fiber. Pedestal-free pulses of 15 µJ are generated after pulse compression at a repetition rate of 500 kHz, and the pulse width is 300 fs. When small-core-size fibers are used at the final amplifier, the generation of spiky modulations in amplified spectra is observed. We analyze the physical origin of the spiky spectral modulation and investigate the effect of the spectral modulation on the pulse shape and contrast after pulse compression by numerical simulations.

Compact, robust, and repetition-rate-locked all-polarization-maintaining femtosecond fiber laser system

We demonstrate a 200-MHz all polarization-maintaining, repetition-rate-locked femtosecond fiber laser system with a total electrical power consumption of 11 W. The center wavelength, spectral width, pulse width, and average output power of the laser are 1558.8 nm, 34 nm, 139 fs, and 77.6 mW, respectively. The proposed laser system that integrates all optical components and locking electronics has a volume less than 1.5 L, a mass of 1.3 kg, and a fast locking time of 3 s (from the free running state to the repetition-rate-locked state). Using a hydrogen maser as the frequency reference, after locking, the Allan deviation is 2.8 mHz at a gate time of 1 s. Further, we place the repetition-rate-locked fiber laser system on a homemade shaker table with peak and rms accelerations of 1.97 and 0.7 g, respectively; the experimental results show that the locking state can be maintained robustly with Allan deviation of 2.0 mHz. The highly integrated, robust fiber laser system has potential applications in the areas of ultralow-noise microwave generation and high-precision distance measurement in outdoor environments.

Custom fabrication and mode-locked operation of a femtosecond fiber laser for multiphoton microscopy

Solid-state femtosecond lasers have stimulated the broad adoption of multiphoton microscopy in the modern laboratory. However, these devices remain costly. Fiber lasers offer promise as a means to inexpensively produce ultrashort pulses of light suitable for nonlinear microscopy in compact, robust and portable devices. Although encouraging, the initial methods reported in the biomedical engineering community to construct home-built femtosecond fiber laser systems overlooked fundamental aspects that compromised performance and misrepresented the significant financial and intellectual investments required to build these devices. Here, we present a practical protocol to fabricate an all-normal-dispersion ytterbium (Yb)-doped femtosecond fiber laser oscillator using commercially-available parts (plus standard optical components and extra-cavity accessories) as well as basic fiber splicing and laser pulse characterization equipment. We also provide a synthesis of established protocols in the laser physics community, but often overlooked in other fields, to verify true versus seemingly (partial or noise-like) mode-locked performance. The approaches described here make custom fabrication of femtosecond fiber lasers more accessible to a wide range of investigators and better represent the investments required for the proper laser design, fabrication and operation.

926 nm Yb-doped fiber femtosecond laser system for two-photon microscopy

A Yb-doped fiber femtosecond laser system operating at 926 nm for two-photon microscopy is demonstrated. The laser system includes a master oscillator power-amplifier system, and the output wavelength shifts from 1064 nm to 926 nm by employing the nonlinear effect in photonics crystal fiber. The laser system produces 88 fs pulse train with 3.3 nJ pulse energy, 78 MHz repetition and 253 mW output power. The two-photon fluorescent image of a mouse kidney section proves that our femtosecond Yb-doped fiber laser system is an ideal source for two-photon microscopy.

172 fs, 24.3 kW peak power pulse generation from a Ho-doped fiber laser system.

We demonstrate a high-peak-power femtosecond fiber laser system based on single-mode holmium (Ho)-doped fibers. 833fs, 27.7MHz pulses at 2083.4nm generated in a passively mode-locked Ho fiber laser are amplified and compressed to near transform-limited 172fs, 7.2nJ pulses with 24.3kW peak power. We achieve this performance level by using the soliton effect and high-order soliton compression. To the best of our knowledge, this is the first demonstration of sub-200fs pulses, with peak power exceeding 10kW from a Ho-doped single-mode fiber laser system without using bulk optics compressors.

High-energy femtosecond fiber laser system and pulse selection based on high-repetition rate KTiOPO4 Pockels cell

A fiber chirped-pulse amplification system with pulse energy as high as 105 μJ is achieved at 200-kHz repetition rate using the rod-type photonic crystal fiber. The whole system’s nonlinearity accumulated in the fiber amplification is effectively suppressed, and the compressed pulse duration of 808 fs is obtained. A 500-kHz high-repetition rate KTiOPO4 Pockels cell is also applied to make the ultrafast laser pulse selection for generating pulse trains with controllable pulse number and pulse splitting without changing the pulse energy. The demonstrated pulse selection and splitting method are useful for processing of different materials and parallel processing. The pulse selection efficiency of the Pockels cell is as high as 96%.

Periodic disruptions induced by high repetition rate femtosecond pulses on magnesium-oxide-doped lithium niobate surfaces

In this paper, we demonstrate the periodic disruption formation on magnesium-oxide-doped lithium niobate surfaces by a femtosecond fiber laser system with wavelength and repetition rate of 1040 nm and 52 MHz, respectively. Three main experimental conditions, laser average power, scanning speed, and orientation of sample were systematically studied. In particular, the ablation morphologies of periodic disruptions under different crystal orientations were specifically researched. The result shows that such disruptions consisting of a bamboo-like inner structure appears periodically for focusing on the surface of X-, Y- and Z-cut wafers, which are formed by a rapid quenching of the material. Meanwhile, due to the anisotropic property, the bamboo-like inner structures consist of a cavity only arise from X- and Z-cut orientation.