Femtosecond Cr(4+):YAG laser with an L-fold cavity operating at a 1.2-GHz repetition rate

Pulses of 55-fs width and 1.2-GHz repetition rate at 1.52 mum are generated from an L-fold cavity Cr(4+):YAG laser by optimization of the lasing-beam mode inside the gain medium. The pulse width is comparable with that obtained from a standard Z-fold cavity. The pulses are characterized by a second-harmonic-generation frequency-resolved optical gating method. The pulse shape deviates from a sech(2) function because of its broad shoulders. With further miniaturization it should be possible to extend the Cr(4+):YAG mode-locked operation to multigigahertz rates.     

High-energy self-starting femtosecond Cr(4+):Mg(2)SiO(4) oscillator operating at a low repetition rate

We describe a self-starting high-power femtosecond laser based on the Cr(4+):Mg(2)SiO(4) crystal that produces 17-nJ pulses of 40-fs duration at 26.5-MHz repetition rate. This low repetition rate is achieved by employment of a one-to-one telescope in the cavity. The pulse energy is five times greater than with a short-resonator laser. To our knowledge, the laser produces the highest energy ever achieved from this type of laser directly from the resonator without cavity dumping or external amplification. We believe that this laser source can be used for many applications, including nonlinear optics, microscopic imaging, and micromachining of silicon and other semiconductor materials.     

Self-starting soliton-modelocked femtosecond Cr(4+):YAG laser using an antiresonant Fabry–Pérot saturable absorber

Received: 31 January 1997     

True fundamental solitons in a passively mode-locked short-cavity Cr(4+):YAG laser

We demonstrate a self-starting, passively mode-locked short-cavity Cr(4+):YAG laser that supports fundamental intracavity solitons over wide ranges of cavity group-velocity dispersion and pulse energies. The total dispersion and nonlinear effects are small enough that stable, N=1 soliton pulses are generated. Equally spaced multiple pulsing is also observed, with fundamental soliton behavior preserved. Regions of bistability exist where, at a constant cavity dispersion, the laser can produce transform-limited pulses of a different width and energy. The laser produces 200-fs pulses at approximately 0.9-, 1.8-, and 2.7-GHz repetition rates with a total of 82 mW of average output power.     

Generation of 20-fs pulses by a prismless Cr(4+):YAG laser

Ultrafast optical pulses shorter than 20 fs with 400-mW average power at a 110-MHz repetition rate have been generated by a Cr(4+):YAG laser with only double-chirped mirrors for dispersion compensation. The corresponding pulse spectrum has a peak intensity at 1450 nm and extends from 1310 to 1500 nm full width at half-maximum (FWHM). These pulses, which are believed to be the shortest generated to date from a Cr(4+):YAG laser, are only four optical cycles within the FWHM intensity width.     

Self-starting mode-locked Cr(4+):YAG laser with a low-loss broadband semiconductor saturable-absorber mirror

We report a self-starting mode-locked Cr(4+):YAG laser with a low-loss broadband metal reflector-based semiconductor saturable-absorber mirror. The minimum pulse width was 44 fs at a wavelength of 1520 nm.     

Stretched-pulse mode-locking synchronously triggered by a femtosecond master laser at sub-megahertz repetition rate

A stretched-pulse mode-locked ytterbium-doped fiber laser was passively synchronized to a femtosecond Ti:sapphire laser at a low repetition rate of 240 kHz through large cross absorption modulation along additional 1-m-long erbium-doped fiber. The synchronous fiber laser with an ultra-long fiber cavity could produce not only nanosecond flat-top pulses with tunable pulse duration but also Gaussian-shape stretched pulses with its minimum pulse duration of ∼450 ps as confirmed by cross-correlation measurement. When operating in the stretched pulse regime, the sub-nanosecond fiber laser could be synchronously triggered by the master injection with the cavity-length mismatch tolerance up to ∼7.8 cm and timing jitter less than 400 fs, confirming that the stretched-pulse mode-locking of the ultra-long slave fiber laser could be robustly controlled by cross absorption modulation effects in the erbium-doped fiber with appropriate femtosecond master injection.     

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr(4+):forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.     

550 MHz carbon nanotube mode-locked femtosecond Cr:YAG laser

We demonstrate a femtosecond Cr:YAG laser mode-locked by a carbon nanotube saturable absorber mirror(CNT-SAM) at a repetition rate of 550 MHz. By employing the CNT-SAM, which exhibits a modulation depth of 0.51% and a saturation fluence of 28 μJ∕cm~2 at 1.5 μm, we achieved a compact bulk Cr:YAG laser with selfstarting mode-locked operation near 1.5 μm, delivering an average output power of up to 147 m W and a pulse duration of 110 fs. To our knowledge, this system provides the highest repetition rate among reported CNT-SAM mode-locked Cr:YAG lasers and the shortest pulse duration among saturable absorber mode-locked Cr:YAG lasers with repetition rates above 500 MHz.     

30 mJ, TEM00, high repetition rate, mechanically Q-switched Er:YAG laser operating at 2940 nm

The 2940 nm Er:YAG laser Q-switched mechanically by means of a rotating mirror was developed. It generated the output pulses of up to 30 mJ energy, below 300 ns duration and record repetition rate of 25 Hz. The developed laser was effectively used for the investigation of laser beam interaction with selected organic matter simulants.     

Diode-pumped self-Q-switched single-frequency 946-nm Nd(3+), Cr(4+):YAG microchip laser

A stable low-threshold self-Q-switched diode-pumped 946-nm Nd(3+), Cr(4+):YAG microchip laser operating in single frequency and fundamental transverse mode is reported. The output characteristics of the microchip laser were investigated under cw and pulsed pumping. By combination of the infrared microchip and an external frequency doubler, 473-nm pulses with a conversion efficiency of 18% were achieved.     

High-power diode-pumped Q-switched and mode-locked Nd:YVO(4) laser with a Cr(4+):YAG saturable absorber

We demonstrate a high-power passively Q -switched and mode-locked Nd:YVO(4) laser with a Cr(4+):YAG saturable absorber. 2.7 W of average power with an 18-kHz Q -switched repetition rate was generated at a 12.5-W pump power. The peak power of a single pulse near the maximum of the Q -switched envelope was greater than 100 kW.     

Generation of a broadband continuum by a Ti:sapphire femtosecond oscillator with a 1-GHz repetition rate

A five-element Ti:sapphire femtosecond ring oscillator emitting a broadband continuum that ranges from 560 to 1150 nm at -50 dB below the maximum with a repetition rate of 1 GHz is demonstrated. The key element is a slightly convex cavity mirror that increases the self-amplitude modulation of a short pulse inside the resonator. Flat negative intracavity group-delay dispersion is required only for the core spectral part of the pulse. We believe that the device presented will make optical frequency metrology and future optical atomic clocks simpler and more stable. Within the reported ultrabroad spectrum a distinct strong emission band near 655 nm occurs that can be extracted to a powerful femtosecond pulse source far out of the amplification of Ti:sapphire.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

Kerr-lens mode-locked polycrystalline Cr:ZnS femtosecond laser pumped by a monolithic Er:YAG laser

We demonstrated a Kerr-lens mode-locked polycrystalline Cr:ZnS laser pumped by a narrow-linewidth linearpolarised monolithic Er:YAG nonplanar ring oscillator operated at 1645 nm. With a 5-mm-thick sapphire plate for intracavity dispersion compensation, a compact and stable Kerr-lens mode-locking operation was realised. The oscillator delivered 125-fs pulses at 2347 nm with an average power of 80 m W. Owing to the special polycrystalline structure of the Cr:ZnS crystal, the second to fourth harmonic generation was observed by random quasi-phase-matching.     

In-line Nd:YAG resonator using self-pumped phase conjugation in Cr(4+):YAG

We report the operation of an in-line Nd:YAG resonator using self-pumped phase conjugation in Cr(4+): YAG at lambda=1.064microm in the nanosecond regime. The output beam has a high-quality diffraction-limited TEM(00) mode and a pulse shape of ~15ns , showing the self- Q -switching ability of the resonator. A maximum extraction of 18mJ and a maximum reflectivity of 2.0 are measured. A transient model provides insight into the energy characteristics and the dynamics of such an in-line resonator.     

High repetition rate laser pulses amplified by Nd/Cr:YAG ceramic amplifier under CW arc-lamp-light pumping

Laser pulses with high repetitive rate generated from a Q-switch microchip Nd:YAG oscillator were amplified by Active mirror composed of Nd/Cr:YAG ceramic pumped by CW arc-lamp. The laser pulses were amplified, and saturation of averaged output laser power was observed. Repetitive ratio of injected laser pulses was changed from 20 to 100 kHz. Averaged output laser power and gain were measured, and gain coefficient and power-extraction efficiency were evaluated. Output laser power was calculated and compared to experimentally measured one, and the calculated results are consistent with the experimental ones. For amplification of laser pulses with high repetitive ratio, this laser material can realize very high laser gain with low pumping power density and high optical-optical conversion efficiency under CW-lamp-light pumping.     

Optical breakdown and filamentation of femtosecond laser pulses propagating in air at a kHz repetition rate

We report the experiments on the optical breakdown and filamentation of femtosecond laser pulses propagating in air at a kHz repetition rate and with several hundreds micro-joule-energy.A 10m-long filament and its breakup and merging at the nonlinear focal region produced by modulational instability of femtosecond laser pulses in air are observed.A simple model based on the nonlinear Schroedinger equation coupled with multiphoton ionization law is presented to explain the several experimental results.     

303MHz高重复频率掺Er光纤飞秒激光器

高重复频率掺Er光纤飞秒激光器在光学频率梳、超高速光学采样等领域具有很重要的作用. 本文采用非线性偏振旋转锁模机理, 在掺Er光纤飞秒激光器中实现了重复频率为303 MHz的锁模脉冲输出. 通过优化腔内色散, 激光器腔内色散在零色散附近偏负值, 锁模后工作在展宽脉冲锁模状态. 在817 mW抽运功率下, 激光器在连续光状态下可以输出125 mW的平均功率, 在锁模状态下可以输出69 mW的平均功率, 脉冲宽度为90 fs. 当抽运功率处于700-817 mW时, 激光器可以实现自启动锁模. 激光器重复频率在5 h内的漂移量为30 Hz.     

100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate

We developed a compact three-stage Ti:sapphire amplifier laser system that produced peak power in excess of 100 TW for a pulse duration of less than 19 fs and an average power of 19 W at a 10-Hz repetition rate. A final 40-mm-diameter Ti:sapphire amplifier is pumped by a Nd:YAG master-oscillator-power-amplifier system that produces ~7-J output of 532-nm radiation. The spatial beam quality is approximately 2 times diffraction limited for the full amplified compressed output pulse. With f/3 optics, this system should therefore be capable of producing a focused intensity of ~3x10(20) W/cm(2) .