Mode-locked Femtosecond Laser Research Articles

Ultrafast, sub-nanometre-precision and multifunctional time-of-flight detection

Displacement measurement is a fundamental functionality in modern science and technology. Although there has been remarkable progress in the precision of such measurements with various laser ranging methods1–8, they are incapable of capturing fast and complex mechanical displacements. Here, we have established a displacement measurement method using time-of-flight detection9 with femtosecond optical pulses and frequency-locked electrical waveforms. This method uniquely combines ultrafast measurement speed, sub-nanometre precision and non-ambiguity range of more than several millimetres. The achieved performance features unprecedented detection speed and precision. Starting from 24 nm precision for 4 ns acquisition time, the precision can reach 180 pm for 5 ms acquisition time. Using this method, we show real-time detection of single-event, fast and high-dynamic-range mechanical displacements. This capability can lead to the realization of new measurement and analysis platforms for studying broadband, transient and nonlinear mechanical dynamics in real time, which will be useful for directly probing optomechanics10, the onset of cracks11, dynamic deformations12, nonlinear vibrations13, ultrasonic phenomena14 and cell-generated forces15. Using a femtosecond mode-locked laser and a frequency-locked electric signal, a displacement measurement method that offers a >MHz measurement speed, sub-nanometre precision and a measurement range of more than several millimetres is achieved, facilitating the study of broadband, transient and nonlinear mechanical dynamics in real time.

Smooth and flat photoluminescence spectra of Nd3+ active ions in tri-doped CaF2 single crystals

Spectroscopic properties of tri-doped Nd3+,R13+,R23+:CaF2 (R1, R2=Y, La, Gd, Lu) crystals were investigated, where R13+ and R23+ act as the lattice regulating ions. According to the spectra properties of co-doped Nd3+,R3+:CaF2 (R = Y, La, Gd, Lu) crystals, the regulating ions are classified into two categories, Y3+ and Lu3+ ions are in the same class while La3+ and Gd3+ are divided into the other. Attribute to the “combination effect” of the regulating ions, the absorption spectra can be regulated controllably in tri-doped CaF2 crystals, and the emission spectra of Nd3+ in CaF2 crystals become smoother and flatter by incorporating two different types of regulating ions. Preliminary CW laser experiment was conducted for Nd3+,Y3+,Gd3+:CaF2 crystal and Nd3+,Lu3+,Gd3+:CaF2 crystal. The slope efficiency and maximum output power of the former reached up to 49.4% and 2.1W, respectively. A CW wavelength tuning range of 34 nm (1043-1077 nm) was obtained for Nd3+,Y3+,Gd3+:CaF2 crystal. The spectra properties and CW tuning operation indicate that the tri-doped Nd3+,R13+,R23+:CaF2 crystals are suitable for the application of femtosecond mode-locked laser.

VSe2 nanosheets for ultrafast fiber lasers

In this work, we investigate VSe2 for generating femtosecond and large energy mode-locked laser pulses for the first time.

A New Optical Angle Measurement Method Based on Second Harmonic Generation with a Mode-Locked Femtosecond Laser

This paper proposes a new optical angle measurement method based on second harmonic generation with a mode-locked femtosecond laser source by taking use of the unique characteristic of high peak power and high-intensity electric field of the femtosecond laser pulses. The angle is measured from the second harmonic wave intensity, which is a function of the angle between the laser beam axis and the optic axis of a nonlinear crystal attached to the target of interest. It is found that the beta barium borate (BBO) crystal is suitable as the nonlinear crystal for this purpose through theoretical simulation. As the first step of research, an experimental demonstration is also carried out in such a way that a change of the second harmonic wave intensity due to the angular displacement of BBO crystal is measured to verify the feasibility of the proposed principle of the angle measurement method.

Optical parametric chirped pulse amplifier producing ultrashort 10.5 mJ pulses at 1.55 μm.

We present an optical parametric chirped pulse amplifier (OPCPA) delivering 10.5 mJ pulses with durations down to 220 fs, at 100 Hz repetition rate, centered at 1550 nm. The system is pumped by a picosecond Nd:YAG amplifier at 1064 nm based on quasi-continuous-wave diode pumping and seeded by a femtosecond mode-locked Er fiber laser at 1550 nm. This choice of wavelengths enables the use of well-established technology and optical components for both pump and signal beams, resulting in a straightforward and robust system design and the ability for further power scaling to be used in high-energy laser-produced plasma experiments.

Investigation and Improvement of Thermal Stability of a Chromatic Confocal Probe with a Mode-Locked Femtosecond Laser Source

An intentional investigation on the thermal stability of a mode-locked femtosecond laser chromatic confocal probe, which is a critical issue for the probe to be applied for long-term displacement measurement or surface profile measurement requiring long-time scanning, is carried out. At first, the thermal instability of the first prototype measurement setup is evaluated in experiments where the existence of a considerably large thermal instability is confirmed. Then the possible reasons for the thermal instability of the measurement setup are analyzed quantitatively, such as the thermal instability of the refractive index of the confocal lens and the thermal expansion of mechanical jigs employed in the probe. It is verified that most of the thermal instability of the measurement setup is caused by the thermal expansion of mechanical jigs in the probe. For the improvement of the thermal stability of the probe, it is necessary to employ a low thermal expansion material for the mechanical jigs in the measurement setup and to shorten the optical path length of the laser beam. Based on the analysis result, a second prototype probe is newly designed and constructed. The improved thermal stability of the second prototype probe is verified through theoretical calculations and experiments.

L-band mode-locked femtosecond fiber laser with gigahertz repetition rate.

We demonstrate an L-band passively mode-locked femtosecond fiber laser with a fundamental repetition rate of 1.013GHz based on a semiconductor saturable absorber mirror. Compared with other reported L-band fiber lasers that use overlong Er-doped fiber, the laser here consists of 10cm heavily doped fiber to increase the fundamental pulse repetition rate to gigahertz level. An output ratio as low as 0.2% is used to make the central wavelength up to 1.6μm. The laser operates at the soliton regime with a 3dB spectral bandwidth of 13.6nm and a pulse duration of 229fs.

Mode-locked femtosecond laser with a rotary Nd:glass disk.

We demonstrate here femtosecond pulses output from a rotary Nd:glass disk laser, mode-locked with semiconductor saturable absorber mirrors. We obtained an average output power of 0.66W in cw operation and an average output power of 0.49W with a duration of 324fs in mode-locked operation in the case of an absorbed pump power of 7.3W. To the best of our knowledge, it is the first time that sub-picosecond pulses have been obtained from rotary Nd:glass disk lasers. It shows a new concept for all-solid-state mode-locked glass lasers with high power.

All-polarization-maintaining, semiconductor saturable absorbing mirror mode-locked femtosecond Er-doped fiber laser with a gigahertz fundamental repetition rate

A passively mode-locked fiber laser with a 1.03 GHz fundamental repetition rate and a low noise performance is demonstrated. The compact and robust laser operates at a central wavelength of 1553.9 nm with a 3 dB spectral bandwidth of 7.9 nm and a temporal width of 550 fs. All the fibers and components used in the laser are polarization-maintaining (PM), so the output pulses are linearly polarized. The degree of polarization of the optical pulses is measured to be 0.9994. The phase noise of the seventh harmonic (7.21 GHz) is measured and a low timing jitter of 11.7 fs is obtained by integrating the phase noise from 30 MHz down to 100 Hz. The noise performance is analyzed in detail, and the methods for further optimizing the timing jitter are also proposed. So far, we know this to be the first-reported low-noise all-PM erbium-doped fiber laser with a gigahertz level pulse repetition rate.

AN INSTRUMENTAL BASIS FOR MULTISPECTRAL LIDAR WITH SPECTRALLY-RESOLVED DISTANCE MEASUREMENTS

Abstract. Hyperspectral solutions augment laser scanning technology with material probing capabilities by measuring target reflectance along with topography. We propose a novel instrumental basis that enables also spectrally-resolved distance measurement with sufficient sensitivity as to access dispersive phenomena on the reflecting target and along the propagation medium, further enhancing the material analysis capabilities of hyperspectral approaches. To this end we have extended distance metrology using intermode beat notes of a mode-locked femtosecond laser to cover flexibly selected spectral regions. The approach is based on an ultra-broadband source derived from a femtosecond laser via coherent supercontinuum generation. Herein we provide a first demonstration of the successful application of this technique to reflectorless measurements and thus its feasibility for multispectral LiDAR. We use a table-top experimental set-up to assess the approach by measuring distance, spectrally-resolved relative distance and reflectance to 5 different material samples on 5 multiplexed contiguous spectral bands of 50 nm in the range of 600 nm to 850 nm. We have achieved a distance measurement precision and accuracy better than 100 μm using integration times of about 30 ms, with promising perspectives regarding scalability to practical distances. The spectrally-resolved distance measurements also show repeatable material-dependent profiles with differences between materials up to several tenths of mm in some spectral bands. Combined with simultaneously acquired reflectance estimations, these profiles enable collecting additional target information, indicating the potential of the approach to enhance the material probing capabilities of prospective multispectral laser scanners.

Tuning thermo-optic properties of BF and NR dyes using plasmonic silver nanoparticles

The linear and thermo-optical properties of two dyes-Basic Fuchsin and Neutral Red were tailored using plasmonic silver nanoparticles. The silver nanosol was synthesized using a mode-locked femtosecond laser ablation of a silver target. The synthesized nanoparticles were having an average size of 7 nm. The results were interpreted in terms of dye-nanoparticle interaction and the formation of dye-nanoparticle resonance hybrids.

MXene-based saturable absorber for femtosecond mode-locked fiber lasers.

We report simple and compact all-fiber erbium-doped soliton and dispersion-managed soliton femtosecond lasers mode-locked by the MXene Ti3C2Tx. A saturable absorber device fabricated by optical deposition of Ti3C2Tx onto a microfiber exhibits strong saturable absorption properties, with a modulation depth of 11.3%. The oscillator operating in the soliton regime produces 597.8 fs-pulses with 5.21 nm of bandwidth, while the cavity with weak normal dispersion (~0.008 ps2) delivers 104 fs pulses with 42.5 nm of bandwidth. Our results contribute to the growing body of work studying the nonlinear optical properties of MXene that underpin new opportunities for ultrafast photonic technology.

Raman-assisted broadband mode-locked laser

The pulse duration that is available from femtosecond mode-locked lasers is limited by the emission bandwidth of the laser crystals used. Considerable efforts have been made to broaden the emission gain bandwidth in these lasers over the past five decades. To break through this limitation, intracavity spectral broadening is required. Here, we propose a new spectral broadening method inside the mode-locked cavity based on use of stimulated Raman scattering and demonstrate significant pulse shortening using this method. We configured Kerr-lens mode-locked lasers based on Yb:CaGdAlO4, Yb:KY(WO4)2 and Yb:Y2O3 materials and achieved significant spectral broadening that exceeds the emission bandwidth. The spectral broadening in the Yb:CaGdAlO4 oscillator shortens the pulse duration to 22 fs, which is a one-third of the duration of our unbroadened mode-locked pulse. The results presented here indicate that Raman-assisted spectral broadening can break the limitations of the emission gain bandwidth and shorten the duration of pulses from femtosecond mode-locked lasers.

Compact low-noise passively mode-locked Er-doped femtosecond all-fiber laser with 2.68 GHz fundamental repetition rate.

A passively mode-locked Er-doped fiber laser with a fundamental repetition rate of 2.68GHz is reported. The oscillator operating at a central wavelength of 1558.35nm has a compact, robust structure and low-noise performance. The timing jitter integrated from 30MHz down to 300Hz is 82.5fs, and the timing jitter performance is analyzed based on the theory model. The amplification and compression of the high repetition rate optical pulses are also investigated. After a three-stage amplifier, the average power is boosted to 430mW. Meanwhile, based on the nonlinear self-phase modulation effect, the spectral bandwidth is broadened from 7.56 to 19.2nm, and the corresponding pulse width is compressed to 244fs.

A Method for Expansion of Z-Directional Measurement Range in a Mode-Locked Femtosecond Laser Chromatic Confocal Probe

A method is proposed to expand the Z-directional measurement range of a fiber-based dual-detector chromatic confocal probe with a mode-locked femtosecond laser source. In the dual-detector chromatic confocal probe, the Z-directional displacement of a measurement target is derived from the peak wavelength in the normalized intensity ratio from the two light intensities obtained by the two identical fiber detectors. In this paper, a new method utilizing the main-lobe and side-lobes of axial responses acquired from both the normalized intensity ratio Ia and the invert normalized intensity ratio In, which is the inverse of Ia, is proposed to obtain the seamless relationship between the peak wavelength and the Z-directional displacement of a measurement target. Theoretical calculations and experimental investigation are carried out to demonstrate the feasibility of the proposed measurement range expansion method.

Ultrafast Dy3+:fluoride fiber laser beyond 3 μm.

We report a passively mode-locked Dy3+:fluoride fiber laser emitting around 3.1μm based on the nonlinear polarization evolution technique in a ring configuration, using in-band pumping at 2.8μm. Transform-limited and self-starting mode-locked pulses as short as 828fs with a center wavelength around 3.1μm and repetition rates up to 60MHz are obtained. In the single-pulse regime, a maximum average output power of 204mW is measured, corresponding to a peak power of 4.2kW and a pulse energy of 4.8nJ. This first demonstration, to the best of our knowledge, of a femtosecond mode-locked fiber laser emitting directly beyond 3 μm paves the way for frequency comb synthesis in the molecular fingerprint region.

Sub-shot-noise interferometric timing measurement with a squeezed frequency comb

Accurate time transfer has become a crucial issue for future space experiments which require increasing resolution over large distances. In 2008, a scheme combining homodyne detection and mode-locked femtosecond lasers was proposed that leads to a potential timing precision reaching the yoctosecond range; with a multimode quantum frequency comb as the input field, the sub-shot noise measurement for the time transfer can further improve the timing precision. Based on this scheme and applying the multimode squeezing frequency comb that was measured to have a phase quadrature quantum noise reduction of 1.5 dB at the analyzing frequency 2MHz, the measurable timing fluctuation was reduced from a shot-noise limited value of 8.9E-23s to 7.5E-23s. To our knowledge, this is the first experimental demonstration of the time transfer measurement that achieves a precision beyond the standard quantum limit (SQL).

Carboxyl graphene oxide solution saturable absorber for femtosecond mode-locked erbium-doped fiber laser**Project supported by the Central University Special Fund Basic Research and Operating Expenses, China (Grant No. GK201702005), the Natural Science Foundation of Shaanxi Province, China (Grant No. 2017JM6091), the National Natural Science Foundation of China (Grant No. 61705183), and the Fundamental Research Funds for the Central Universities, China (Grant

The carboxyl-functionalized graphene oxide (GO-COOH) is a kind of unique two-dimensional (2D) material and possesses excellent nonlinear saturable absorption property and high water-solubility. In this paper, we prepare saturable absorber (SA) device by depositing GO-COOH nanosheets aqueous solution on a D-shaped fiber. The modulation depth (MD) and saturable intensity of the SA are measured to be 9.6% and 19 MW/cm2, respectively. By inserting the SA into the erbium-doped fiber (EDF) laser, a passively mode-locked EDF laser has been achieved with the spectrum center wavelength of 1562.76 nm. The pulse duration, repetition rate, and the signal-to-noise ratio (SNR) are 500 fs, 14.79 MHz, and 80 dB, respectively. The maximum average output power is measured to be 3.85 mW. These results indicate that the GO-COOH nanosheets SA can be used as a promising mode locker for the generation of ultrashort pulses.

Flexible double-cladding ytterbium fibre based 9 W mode-locked laser with 102 fs compressible pulse duration

We report on a high power mode-locked femtosecond fiber laser oscillator with flexible double-cladding (DC) ytterbium-doped fiber (Yb-fiber) as gain medium and a fused fiber combiner as a pump coupling scheme, based on the nonlinear polarization evolution effect in a ring cavity configuration with a dual-stage birefringent plate as the spectral filter. Laser pulses with a pulse energy as high as 122 nJ and compressible pulse duration as short as 102 fs at 1041 nm center wavelength could been obtained. The experimentally recorded power fluctuations were better than 0.42% RMS and 0.40% RMS in 3 h under 5 W and 9 W output power, respectively. The measured beam quality factors M2 were 1.3 in both the horizontal and perpendicular directions. To the best of our knowledge, this is the highest output power generated from a flexible DC Yb-fiber-based ultrashort pulse mode-locking fiber laser oscillator pumped with a fusion spliced combiner.

Continuous repetition rate tuning from 960 MHz to 1.72 GHz of a sub-300 femtosecond mode-locked semiconductor disk laser

Dataset supports: Chen Sverre, T. et al (2018). Continuous repetition rate tuning from 960 MHz to 1.72 GHz of a sub-300 femtosecond mode-locked semiconductor disk laser. Applied Physics Letters, 113(16), [161106].Vertical External Cavity Surface Emitting semiconductor Lasers rely on Semiconductor Saturable Absorbing Mirrors for mode-locking, allowing laser cavities to be designed far from stability limits. We have harnessed this feature to study repetition rate tunability. The cavity element separations were determined by a coded cavity design protocol based on cavity round trip matrix calculations. We produced a sub-300-fs near transform-limited pulse train with a repetition rate of 0.96 MHz - 1.72 GHz, at 1035 nm and average power of 50 mW.