High power optical frequency comb with 10-19 frequency instability.

Optical frequency combs with more than 10 W have paved the way for extreme ultraviolet combs generation by interaction with inert gases, leading to extreme nonlinear spectroscopy and the ultraviolet nuclear clock. Recently, the demand for an ultra-long-distance time and frequency space transfer via optical dual-comb proposes a new challenge for high power frequency comb in respect of power scaling and optical frequency stability. Here we present a frequency comb based on fiber chirped pulse amplification (CPA), which can offer more than 20 W output power. We further characterize the amplifier branch noise contribution by comparing two methods of locking to an optical reference and measure the out-of-loop frequency instability by heterodyning two identical high-power combs. Thanks to the low noise CPA, reasonable locking method, and optical path-controlled amplifiers, the out-of-loop beat note between two combs demonstrates the unprecedented frequency stability of 4.35 × 10-17 at 1s and 6.54 × 10-19 at 1000 s.

2-GHz watt-level Kerr-lens mode-locked Yb:KGW laser.

We report on a 2-GHz high-power Kerr-lens mode-locked Yb:KGW laser pumped by a single-mode fiber laser. The output performance for two different output coupling rates was investigated. Stable bidirectional mode-locking operation at the repetition rate of 2.157 GHz was obtained with a 0.6% output coupler. The average output powers of bidirectional operation are 741 mW and 746 mW, with 123-fs and 126-fs pulse durations, respectively. By using a 1.6% output coupler, unidirectional mode-locking is achieved with 145-fs pulse duration and 1.7-W average output power, which, to the best of our knowledge, is the highest average power from Kerr-lens mode-locked GHz femtosecond oscillators.

A comparison of the multimodal magnetic resonance imaging features of brain metastases vs. high-grade gliomas.

OBJECTIVE: We aimed to explore the multimodal magnetic resonance imaging (MRI) features of brain metastases and high-grade brain gliomas. METHODS: Fifty patients with brain metastases and 28 patients with high-grade gliomas treated in the neurosurgery department of our hospital were selected for this study. All patients underwent routine MRI, diffusion tensor imaging, and perfusion-weighted magnetic resonance imaging. The average diffusion coefficient (ADC), fractional anisotropy (FA), regional cerebral blood flow (rCBF), and regional cerebral blood volume (rCBV) in the tumor parenchyma, peritumoral edema area, and the contralateral normal cerebral white matter were compared between the patients with brain metastases and the patients with high-grade brain gliomas. RESULTS: There were differences in the degree of peritumoral edema between the two groups of patients (P = 0.017). Compared with the patients with high-grade gliomas, the patients with brain metastases had lower FA values in the tumor parenchyma area, higher ADC values in the peritumoral edema area, and lower rCBV and rCBF values in the peritumoral edema area (all P<0.001). CONCLUSION: The measurement of the ADC, rCBV, and rCBF values in the peritumoral edema area and the FA values in the tumor parenchyma area using multimodal MRI can have essential clinical value in the differentiation between brain metastases and high-grade gliomas.

Femtosecond-Laser-Ablation Dynamics in Silicon Revealed by Transient Reflectivity Change.

The dynamics of ablation in monocrystalline silicon, from electron-hole plasma generation to material expansion, upon irradiation by a single femtosecond laser pulse (1030 nm, 300 fs pulse duration) at a wide range of fluences is investigated using a time-resolved microscopy technique. The reflectivity evolution obtained from dynamic images in combination with a theoretical Drude model and a Two-Temperature model provides new insights on material excitation and ablation process. For all fluences, the reflectivity increased to a temporary stable state after hundreds of femtoseconds. This behavior was predicted using a temperature-dependent refractive index in the Drude model. The increase in velocity of plasma generation with increasing fluence was theoretically predicted by the Two-Temperature model. Two ablation regimes at high fluences (>0.86 J/cm2) were identified through the measured transient reflectivity and ablation crater profile. The simulation shows that the fluence triggering the second ablation regime produces a boiling temperature (silicon, 2628 K).

Ultra-low-noise carrier-envelope phase stabilization of a Kerr-lens mode-locked Yb:CYA laser frequency comb with a feed-forward method.

We demonstrate ultra-low-noise carrier-envelope phase stabilization of a Kerr-lens mode-locked Yb:CYA laser frequency comb, which is the first time, to the best of our knowledge, that the feed-forward method has been applied to 1 µm all-solid-state lasers. We obtain a signal-to-noise ratio of more than 38 dB at a 100 kHz resolution bandwidth for carrier-envelope phase offset beat signals in two standard in-loop and out-of-loop f-2f interferometers. The residual integrated phase noise amounts to 79.3 mrad from 1 Hz to 1 MHz, corresponding to a timing jitter of 44 as. We also investigate long-term performances of the CEP stabilization in the feed-forward scheme, phase-locked feedback systems and the combining locking techniques in terms of sub-hertz frequency-resolved phase noise and Allan deviation in 1000 s. The results indicate that, although the feed-forward CEP stabilization method dominates in the range of 1 Hz to 1 MHz Fourier frequency, feedback methods with a time integration effect are superior in sub-hertz Fourier frequency phase noise suppression.

Tungsten disulphide for ultrashort pulse generation in all-fiber lasers.

Tungsten disulphide (WS2), which exhibits excellent saturable absorption properties, has attracted much attention in the applications of photonic devices. In this paper, WS2 is applied for the preparation of a saturable absorber (SA). Using the pulsed laser deposition (PLD) method, WS2 is deposited on the side surface of the tapered fiber. In order to obtain larger non-linearity of the SAs with evanescent wave interaction, the tapered fiber had a smaller waist diameter and longer fused zone. Gold film was deposited on the fiber-taper WS2 SAs to improve their reliability and avoid oxidation and corrosion. Employing the balanced twin-detector method, the modulation depth of the fiber-taper WS2 SAs was measured to be 17.2%. With the fiber-taper WS2 SA, a generated pulse with 246 fs duration and a 57 nm bandwidth was obtained at 1561 nm. The electrical signal to noise ratio was better than 92 dB. To our knowledge, the pulse duration was the shortest among the reported all-fiber lasers with transition metal dichalcogenide (TMD) SAs. These results indicate that fiber-taper WS2 SAs with smaller waist diameter and longer fused zone are promising photonic devices for ultrashort pulse generation in all-fiber lasers.

Tungsten disulfide saturable absorbers for 67 fs mode-locked erbium-doped fiber lasers.

In this paper, we demonstrate 67 fs pulse emitting with tungsten disulfide (WS2) in mode-locked erbium-doped fiber (EDF) lasers. Using the pulsed laser deposition method, WS2 is deposited on the surface of the tapered fiber to form the evanescent field. The fiber-taper WS2 saturable absorber (SA) with the large modulation depth is fabricated to support the ultrashort pulse generation. The influences of the WS2 SA are analyzed through contrastive experiments on fiber lasers with or without the WS2 SA. The pulse duration is measured to be 67 fs, which is the shortest pulse duration obtained in the mode-locked fiber lasers with two dimensional (2D) material SAs. Compared to graphene, topological insulator, and other transition metal dichalcogenides (TMDs) SAs, results in this paper indicate that the fiber-taper WS2 SA with large modulation depth is a more promising photonic device in mode-locked fiber lasers with the wide spectrum and ultrashort pulse duration.

Ultra-stability Yb-doped fiber optical frequency comb with 2 × 10-18/s stability in-loop.

We demonstrate a full control ultra-stability Yb-doped fiber optical frequency comb (OFC). The carrier-envelop offset frequency (fceo) and the repetition rate (fr) are locked with the standard phase locked loop (PLL) technique. The fceo is locked to the radio frequency (RF) synthesizer, and the Allan deviation is 1.2 × 10-17/s. The fr is locked to an ultra-stability continuous wave (CW) laser at 972 nm. The beat signal (fbeat) between the Yb-doped fiber OFC and CW laser is obtained with the signal to noise ratio (SNR) of 43 dB at 300 kHz resolution bandwidth (RBW). The time jitter of the fbeat signal is 278 as, which is integrated from 1 Hz to 10 MHz. The long-term stability is 575 µHz in 3 hours, and the corresponding Allan deviation is 2 × 10-18/s, which is the best stability result in Yb-doped fiber OFC. The linewidth is narrowed from 200 kHz to subhertz magnitude limited by the instrument resolution bandwidth.

CEO stabilized frequency comb from a 1-µm Kerr-lens mode-locked bulk Yb:CYA laser.

We report the first Kerr-lens mode-locked (KLM) bulk frequency comb in the 1-µm spectral regime. The fundamental KLM Yb:CYA laser is pumped by a low-noise, high-bright 976-nm fiber laser and typically provides 250-mW output power and 57-fs pulse duration. Only 58-mW output pulses were launched into a 1.3-m photonic crystal fiber (PCF) for one octave-spanning supercontinuum generation. Using a simplified collinear f-2f interferometer, the free-running carrier-envelope offset (CEO) frequency was measured to be 42-dB signal-to-noise ratio (SNR) for a 100-kHz resolution and 9.6-kHz full width at half maximum (FWHM) under a 100-Hz resolution. A long-term CEO control at 23 MHz was ultimately realized by feeding the phase error signal to the pump power of the oscillator. The integrated phase noise (IPN) of the locked CEO was measured to be 316 mrad with an integrated range from 1 Hz to 10 MHz. The standard deviation and Allan deviation for more than 4-hour recording are 1.6 mHz and 5.6 × 10(-18) (for 1-s gate time), respectively. This is, to the best of our knowledge, the best stability achieved among the 1-µm solid-state frequency combs.

70-fs mode-locked erbium-doped fiber laser with topological insulator.

Femtosecond optical pulses have applications in optical communication, astronomical frequency combs, and laser spectroscopy. Here, a hybrid mode-locked erbium-doped fiber (EDF) laser with topological insulator (TI) is proposed, for the first time to our best knowledge. The pulsed laser deposition (PLD) method is employed to fabricate the fiber-taper TI saturable absorber (TISA). By virtue of the fiber-taper TISA, the hybrid EDF laser is passively mode-locked using the nonlinear polarization evolution (NPE), and emits 70 fs pulses at 1542 nm, whose 3 dB spectral width is 63 nm with a repetition rate and transfer efficiency of 95.4 MHz and 14.12%, respectively. Our experiments indicate that the proposed hybrid mode-locked EDF lasers have better performance to achieve shorter pulses with higher power and lower mode-locking threshold in the future.

Quasi-steady-state air plasma channel produced by a femtosecond laser pulse sequence.

A long air plasma channel can be formed by filamentation of intense femtosecond laser pulses. However, the lifetime of the plasma channel produced by a single femtosecond laser pulse is too short (only a few nanoseconds) for many potential applications based on the conductivity of the plasma channel. Therefore, prolonging the lifetime of the plasma channel is one of the key challenges in the research of femtosecond laser filamentation. In this study, a unique femtosecond laser source was developed to produce a high-quality femtosecond laser pulse sequence with an interval of 2.9 ns and a uniformly distributed single-pulse energy. The metre scale quasi-steady-state plasma channel with a 60-80 ns lifetime was formed by such pulse sequences in air. The simulation study for filamentation of dual femtosecond pulses indicated that the plasma channel left by the previous pulse was weakly affected the filamentation of the next pulse in sequence under our experimental conditions.

Generation of dark solitons in erbium-doped fiber lasers based Sb(2)Te(3) saturable absorbers.

Dark solitons, which have better stability in the presence of noise, have potential applications in optical communication and ultrafast optics. In this paper, the dark soliton formation in erbium-doped fiber lasers based Sb(2)Te(3) saturable absorber (SA) is first experimentally demonstrated. The Sb(2)Te(3) SA is fabricated by using the pulsed laser deposition method. The generated dark solitons are centered at the wavelength of 1530 nm and repetition rate of 94 MHz. Analytic solutions for dark solitons are also obtained theoretically.

All-normal-dispersion passive harmonic mode-locking 220 fs ytterbium fiber laser.

We report a stable passive second-harmonic mode-locked all-normal-dispersion ytterbium fiber laser based on nonlinear polarization evolution. This fiber laser has two polarization beam splitter output ports for optimizing the output spectrum 5 ps duration pulses with 187 mW average power being generated at the harmonic repetition rate of 99.6 MHz. By use of a pair of gratings to extracavity compensate the chirp, the pulse is further compressed to 220 fs. We measured that the peak-to-pedestal extinction of the radio frequency is about 80 dB corresponding to a pulse-to-pulse energy fluctuation of 0.32% and timing jitter of 3.2 ps.

Diode-pumped Kerr-lens mode-locked Yb:LYSO laser with 61fs pulse duration.

A stable diode pumped Kerr-lens mode-locked (KLM) Yb:LuYSiO5 (Yb:LYSO) laser of generating 61 fs pulses at a central wavelength of 1055.4 nm is experimentally demonstrated. This is, to the best of our knowledge, the first demonstration of femtosecond KLM operation in Yb:LYSO laser, and it is believed that 61 fs is the shortest pulse duration ever produced from an Yb-doped orthosilicate laser. The average output power of the mode-locked laser is 40 mW and the repetition rate is 113 MHz.

Tunable deep ultraviolet single-longitudinal-mode laser generated with Ba(1-x)B(2-y-z)O4Si(x)Al(y)Ga(z) crystal.

We report a new nonlinear crystal, Ba(1-x)B(2-y-z)O4Si(x)Al(y)Ga(z), and employ it to a compact 1 kHz single-longitudinal-mode Ti:Sapphire master oscillator power amplifier system for fourth harmonic generation. A maximum output power of 130 mW is obtained in the tunable range of 195-205 nm with linewidth of less than 0.1 pm.

Diode-pumped 88-fs Kerr-lens mode-locked Yb:Y3Ga5O12 crystal laser.

We realized a stable Kerr-lens mode-locked operation in a diode-pumped Yb:Y3Ga5O12 laser. Pulses as short as 88 fs at the center wavelength of 1042 nm were obtained at a repetition rate of 159.3 MHz. The maximum output power was 104 mW under the incident pump power of 3.9 W. By comparing the mode-locked characteristics under different output transmissions, we obtained pulses with the highest output power of 330 mW and a duration of 149 fs. To the best of our knowledge, this is the first demonstration of a Kerr-lens mode-locked Yb:Y3Ga5O12 laser.

Efficient femtosecond optical parametric oscillator with dual-wavelength operation.

We demonstrated on a synchronously pumped femtosecond optical parametric oscillator with dual-signal-wavelength operation. Our results showed that the dual-wavelength oscillation is not determined by the net-zero dispersion but rather by the balance of phase matching and group-velocity mismatching caused by the nonlinear crystal between the two signals. With a MgO-doped periodically poled lithium niobate as the nonlinear crystal, total signal power up to 530 mW was obtained by using a 2.6 W femtosecond Ti:sapphire laser at the central wavelength of 808 nm as the pump source, corresponding to a conversion efficiency of 20.4%. By slightly adjusting the cavity length, the signal wavelength can be broadly tuned from 1001 to 1438 nm and the dual-wavelength tuning range is from 1001 to 1204 nm.

Contrast enhancement in a Ti:sapphire chirped-pulse amplification laser system with a noncollinear femtosecond optical-parametric amplifier.

We experimentally demonstrated the contrast enhancement in a Ti:sapphire chirped-pulse amplification (CPA) laser with a noncollinear femtosecond optical-parametric amplifier. A total gain of 3.4 × 10(4) and pulse energy of 26 µJ were achieved. With the clean high-energy seeding pulse, the contrast ratio of the main amplified laser pulse to the amplified spontaneous emission in the Ti: sapphire CPA laser system was improved to around 10(10) within the time scale of hundreds of picoseconds.

A 350 MHz Ti:sapphire laser comb based on monolithic scheme and absolute frequency measurement of 729 nm laser.

We realized a novel frequency comb based on femtosecond Ti:sapphire laser with 350 MHz repetition rate and monolithic scheme for carrier-envelope-offset frequency generation. Long-term stabilization of more than 9 hours was demonstrated by simultaneously locking repetition rate (f(rep)) and carrier-envelope-offset frequency (f(ceo)) to an atomic clock. The Allan deviations of f(ceo) and f(rep) are 2.3 x 10(-13) and 1.5 x 10(-15) at 100 s averaging time, respectively, and the reproducibility of f(ceo) with high accurate performance is also verified by comparison of the locking results recorded in 3 different days. By avoiding the extra noise induced by photonic crystal fiber, we obtained the accumulated phase error increase from in-loop to out-of-loop as small as 12 mrad [1 Hz, 100 kHz]. Furthermore, the absolute optical frequency of a stabilized 729 nm Ti:sapphire laser which is referenced to a ultra-low expansion (ULE) cavity is measured by the comb and determined to be 411041985391583+/-10 Hz.

Independently tunable 1.3 W femtosecond Ti:sapphire lasers passively synchronized with attosecond timing jitter and ultrahigh robustness.

A stable passively synchronized femtosecond laser has been realized by coupling two 1.3 W mode-locked Ti:sapphire lasers with a Kerr medium. An ultralong tolerance of 10 microm for the cavity length mismatch and a timing jitter of less than 0.4 fs were obtained. The relative carrier-envelope phase slip was directly observed by measuring the heterodyne output between the two lasers.