41-fs, 35-nJ, Green Pulse Generation from a Yb-doped Fiber Laser System.

We report ultrafast green pulse generation from a Yb-doped fiber laser system with gain-narrowing compensation. The chirped-pulse amplification system outputs repetitive 3 MHz pulses with an energy of 35 nJ and a reconstructed pulse duration of 41 fs.

65-fs Yb-doped fiber laser system with gain-narrowing compensation.

We report a broadband Yb-doped fiber laser system with a gain-narrowing compensator comprised of multiple dielectric layers. Utilizing this filter, we obtained broadband pulses over a bandwidth of 1020-1080 nm directly from the amplifier. After the dispersion compensation, the chirped pulse amplification system delivered 65-fs pulses with energies of 100 nJ and a repetition rate of 3 MHz.

Long-term stable passive synchronization between two-color mode-locked lasers with the aid of temperature stabilization.

We demonstrate long-term stable passive synchronization between two-color Ti:sapphire (master) and Yb-doped fiber (slave) mode-locked lasers in the master-slave configuration. Active temperature stabilization suppresses the repetition fluctuation of the slave laser, and with the aid of temperature stabilization in combination with simple repetition locking of the master laser, long-term stable synchronization as long as 6 h was realized. The repetition rates of both lasers are locked in submillihertz precision. A timing jitter of 0.75 fs was obtained at a detection bandwidth of 350 kHz.

Long-term stable passive synchronization of 50 µJ femtosecond Yb-doped fiber chirped-pulse amplifier with a mode-locked Ti:sapphire laser.

We report long-term stable passive synchronization of a femtosecond Yb-doped fiber chirped-pulse amplifier (CPA) with a mode-locked Ti:sapphire laser for pump-seed synchronization of an optical parametric chirped-pulse amplification (OPCPA) system. The fiber CPA system delivers pulses with a wavelength of 1035 nm, energy of 50 µJ, and duration of 690 fs at a repetition rate of 0.4 MHz. The seed fiber oscillator is passively synchronized with a mode-locked Ti:sapphire laser by injection of the Ti:sapphire laser pulses into the cavity of the fiber oscillator. The second harmonic (SH) output with a wavelength of 518 nm, energy of 18 µJ, and duration of 1.2 ps was prepared for the OPCPA pump. The measured timing jitter between the pump (fiber SH) and the seed (Ti:sapphire) was 42 ± 14 fs, while the jitter between two oscillator outputs was 1.4 ± 0.5 fs. The robust synchronization technique allows long-term stable operation over 8 h.

1 W average-power 100 MHz repetition-rate 259 nm femtosecond deep ultraviolet pulse generation from ytterbium fiber amplifier.

We demonstrate 1W average-power ultraviolet (UV) femtosecond (fs) ultrashort pulse generation at a wavelength of 259 nm and a repetition rate as high as 100 MHz by quadrupling a fs ytterbium-fiber laser. A cavity-enhanced design is employed for efficient frequency doubling to the UV region. The optical-to-optical efficiency of UV output to the pump diode is 2.6%.

Fabrication of Hollow Structures in Photodegradable Hydrogels Using a Multi-Photon Excitation Process for Blood Vessel Tissue Engineering.

Engineered blood vessels generally recapitulate vascular function in vitro and can be utilized in drug discovery as a novel microphysiological system. Recently, various methods to fabricate vascular models in hydrogels have been reported to study the blood vessel functions in vitro; however, in general, it is difficult to fabricate hollow structures with a designed size and structure with a tens of micrometers scale for blood vessel tissue engineering. This study reports a method to fabricate the hollow structures in photodegradable hydrogels prepared in a microfluidic device. An infrared femtosecond pulsed laser, employed to induce photodegradation via multi-photon excitation, was scanned in the hydrogel in a program-controlled manner for fabricating the designed hollow structures. The photodegradable hydrogel was prepared by a crosslinking reaction between an azide-modified gelatin solution and a dibenzocyclooctyl-terminated photocleavable tetra-arm polyethylene glycol crosslinker solution. After assessing the composition of the photodegradable hydrogel in terms of swelling and cell adhesion, the hydrogel prepared in the microfluidic device was processed by laser scanning to fabricate linear and branched hollow structures present in it. We introduced a microsphere suspension into the fabricated structure in photodegradable hydrogels, and confirmed the fabrication of perfusable hollow structures of designed patterns via the multi-photon excitation process.

Generation of 28-fs pulses from a mode-locked ytterbium fiber oscillator.

An ultrashort-pulse, mode-locked ytterbium-doped fiber laser has been developed. The group-delay dispersion was compensated with a grating pair inside the cavity. A broad spectrum from 1000-nm to 1120-nm was obtained without intracavity compensation of third-order dispersion. A 0.7-nJ pulse as short as 28.3 fs was obtained with a repetition rate of 80 MHz. To our knowledge, this is the shortest pulse reported from an Yb fiber laser oscillator.

Ultrashort pulse characterization by ultra-thin ZnO, GaN, and AlN crystals.

Ultra-thin semiconductor crystals were investigated as nonlinear materials for second-harmonic generation. Nonlinear susceptibilities of sub-micrometer- thick ZnO, GaN, and AlN crystals were measured, and these crystals were used for sub-10-fs pulse measurement by a fringe-resolved autocorrelation method. We found that a one-cycle pulse could be characterized by using these ultra-thin-film crystals.

Synchronization of Ti:sapphire and Cr:forsterite mode-locked lasers with 100-attosecond precision by optical-phase stabilization.

An optical-phase stabilization technique was utilized to reduce the timing jitter between passively synchronized Ti:sapphire and Cr:forsterite two-color mode-locked lasers. The suppression of cavity-length fluctuation by stabilizing pulse-to-pulse slips of relative carrier-envelope phase allowed timing-jitter reduction by a factor of 1.7, resulting in an rms value of 123 attoseconds (as) in a frequency range from 10 mHz to 1 MHz.

Carrier-envelope-phase stabilized chirped-pulse amplification system scalable to higher pulse energies.

We have demonstrated a carrier-envelope phase (CEP) stabilized chirped-pulse amplification (CPA) system employing a grating-based pulse stretcher and compressor and a regenerative amplifier for the first time. In addition to stabilizing the carrier-envelope offset phase of a laser oscillator, a new pulse selection method referenced to the carrier-envelope offset beat signal was introduced. The pulse-selection method is more robust against the carrier-envelope offset phase fluctuations than a simple pulse-clock dividing method. We observed a stable fringe in a self-referencing spectrum interferometry of the amplified pulse, which implies that the CEP of amplified pulse is stabilized. We also measured the effect of the beam angle change on the CEP of amplified pulses. The result demonstrates that the CEP stabilized CPA is scalable to higher-pulse energies.

Characterization of Fourier-synthesized optical waveforms from optically phase-locked femtosecond multicolor pulses.

For characterization of Fourier-synthesized optical waveforms of femtosecond multicolor phase-coherent pulses, we report relative-phase measurements among Fourier components from optically phase-locked two-color mode-locked lasers. The interference of two simultaneous second-order frequency-mixing processes in a common nonlinear crystal was utilized. The relative phases among three-color components with a frequency ratio of 2omega:3omega:4omega were measured by observing the interference fringes of two simultaneous frequency-mixing outputs. The inner field structure of the synthesized waveforms can be determined except for the ambiguity of its temporal shift with respect to the synthesized envelope, which corresponds to the carrier-envelope phase slip of one of the source lasers.

High-repetition-rate 12 fs pulse amplification by a Ti:sapphire regenerative amplifier system.

We demonstrate the direct generation of 12 fs pulses from a Ti:sapphire regenerative amplifier system at a 1 kHz repetition rate utilizing properly designed broadband components for chirped-pulse amplification. Optimized designs of a regenerative amplifier with a multilayer gain-narrowing compensator and an adaptive dispersion compensator with a spatial light modulator contribute to the shorter pulse amplification.

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train.

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.

Long-term optical phase locking between femtosecond Ti:sapphire and Cr:forsterite lasers.

Long-term optical phase-coherent two-color femtosecond pulses were generated by use of passively timing-synchronized Ti:sapphire and Cr:forsterite lasers. The relative carrier-envelope phase relation was fixed by an active feedback loop. The accumulated phase noise from 10 mHz to 1 MHz of the locked beat note was 0.43 rad, showing tight phase locking. The optical frequency fluctuation between two femtosecond combs was submillihertz, with a 1 s averaged counter measurement over 3400 s, leading to a long-term femtosecond frequency-comb connection.

100-attosecond timing jitter between two-color mode-locked lasers by active-passive hybrid synchronization.

We have demonstrated a reduction of the timing jitter between passively synchronized Ti:sapphire and Cr:forsterite mode-locked lasers into a 100-attosecond (as) regime by suppressing slow fluctuations with the use of active slow-bandwidth extracavity feedback. This active-passive hybrid synchronization scheme permits the achievement of timing jitters of 98 +/- 18 as at a bandwidth of 100 kHz and of 126 +/- 20 as at a bandwidth of 1 MHz for as long as 100 s.

Relative carrier-envelope phase dynamics between passively synchronized Ti:sapphire and Cr:forsterite lasers.

We observed and measured the relative carrier-envelope phase difference per round trip between synchronized femtosecond Ti:sapphire and Cr:forsterite mode-locked lasers. The relative carrier-envelope phase slip was directly recorded by heterodyning of the Cr:forsterite laser with the supercontinuum from the Ti:sapphire laser. We also obtained another phase relation by superimposing the third harmonic of the Cr:forsterite laser with the second harmonic of the Ti:sapphire laser. In the latter case we obtained a stable beat signal with a signal-to-noise ratio larger than 30 dB and found a dependence of the beat frequency on the cavity length.

Control of relative carrier-envelope phase slip in femtosecond Ti:sapphire and Cr:forsterite lasers.

We were able to control relative carrier-envelope phase slip among mode-locked Ti:sapphire and Cr:forsterite lasers by employing electronic feedback. The pulse timings of these lasers were passively synchronized with our crossing-beam technique. Since the optical-frequency ratio of Ti:sapphire and Cr:forsterite is approximately 3:2, we can observe the phase relation by superimposing the third harmonic of Cr:forsterite and the second harmonic of Ti:sapphire lasers in time and in space. The spectrum width of the locked beat note was less than 3 kHz, which corresponds to the controlled fluctuation of a cavity-length difference of less than 10 pm.

Single-shot measurement of a carrier-envelope phase by use of a time-dependent polarization pulse.

We propose a method for single-shot measurement of the carrier-envelope phase of high-intensity laser pulses. The method is based on observation of the electrons' spatial distribution ionized by a time-dependent polarization pulse generated by a combination of replicas of the measuring pulse. The dependence of the electrons' angular distribution on carrier-envelope phase, pulse width, delay between two combining components, and a peak intensity is calculated. Important experimental issues such as broadening of the angular distribution, Gouy phase, difference between the two replicas, and asymmetric pulse shape are also discussed.

Optical phase locking among femtosecond subharmonic pulses.

We stabilized the relative carrier-envelope phase slip among the pump pulse and its subharmonic signal and idler pulses in a femtosecond optical parametric oscillator, resulting in long-term phasecoherence among the pulses. The stabilized beat signal corresponding to the relative carrier-envelope phase slip among subharmonic pulses had an accumulated phase error of 0.24 rad in the 1-mHz-1-MHz region. The fluctuation of the beat frequency measured by a 1-s-averaged counter was less than 1 mHz in a 1480-s measurement.