Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR

Noncollinearly phase-matched optical parametric amplifiers (NOPAs) pumped by the blue light of a frequency-doubled Ti:sapphire regenerative amplifier are a convenient source of continuously tunable ultrashort pulses in the visible and near infrared for spectroscopic experiments. We present the underlying principles, report recent improvements and describe the experiences gained from the routine use of a number of NOPAs in our laboratories. We find that the setup can easily be optimized for the given experimental requirements. Typical output-pulse energies in the visible are 5 to 10 μJ and a few μJ in the NIR from 200 μJ regenerative-amplifier pulses at 800 nm. From 460 to 700 nm, pulse lengths between 10 and 20 fs are routinely achieved, while the length increases monotonically from about 20 fs at 900 nm to just below 50 fs at 1600 nm. In all cases this corresponds to a dramatic shortening compared to the length of the pump pulses of around 100 fs. First results show that the 700 to 900 nm region can be accessed with sub-50-fs pulse lengths by use of an intermediate white-light generator in a two-stage setup. Received: 29 November 1999 / Published online: 5 July 2000     

Intense femtosecond pulse shaping using a fused-silica spatial light modulator

We present the design concept of a setup of a pulse shaper to be used for high-power femtosecond lasers. The pulse shaper is constructed from a high-damage threshold fused-silica spatial light modulator and a 4-f optical system based on the design concept to avoid optical damage. We have successfully demonstrated a pulse compression of 20 fs, 5 mJ pulses obtained from a 1 kHz repetition rate Ti:sapphire chirped pulse amplification system at an average power of 5 W.     

Shaper-assisted full-phase characterization of UV pulses without a spectrometer

A full-phase measurement of low-energy femtosecond UV pulses is presented. The method relies on phase retrieval of measured sonogram traces and is greatly simplified by a two-dimensional shaper-assisted cross correlation setup. As all required pulses are generated by the pulse shaper, the method is free of external references and additional tunable filter setups.     

Independent control over the amplitude,phase, and polarization of femtosecond pulses

We introduce a shaper setup which takes advantage of laser pulses passing through a spatial light modulator twice, thereby effectively utilizing a four-liquid crystal mask configuration. This approach grants control not only over the phase and polarization but also the amplitude. The Jones vector of the light wave after passing through the setup is considered in detail including polarization sensitive grating efficiency. A new method of counteracting the polarization dependent grating transmission is described and a comparison between the desired and recorded pulses is presented. PACS 42.25.Ja; 42.15.Eq; 42.65.Re     

Spectral phase control and temporal superresolution toward the single-cycle pulse

The concept of temporal superresolution is applied to optical few-cycle laser pulses for the first time to our knowledge. Pulse durations of as little as to 3.7 fs, well below the Fourier limit, are achieved by pulse shaping of an octave-spanning Ti:sapphire oscillator spectrum. Our prism-based pulse shaper also enables us to generate a manifold of well-controlled pulse sequences that are important for coherent control applications on a femtosecond time scale.     

Generation of intense highly coherent femtosecond pulses in the mid infrared

Highly coherent mid-infrared femtosecond pulses, tunable between 2.5 and 4.4 mum, and with an average energy and duration between 2.6 and 3.6 microm of 11 microJ and 200fs, respectively, have been produced by an optical parametric amplifier setup driven at 1kHz by a 400- microJ , 800-nm pulse from a Ti:sapphire amplifier. In this system, first tunable moderate-energy femtosecond pulses in the near infrared are produced by continuum amplification in beta -barium borate, and subsequently the near-infrared pulses are amplified and frequency mixed with 800-nm radiation in potassium titanyl phosphate to produce intense tunable mid-infrared pulses. The time-bandwidth product of the mid-infrared pulses is 0.28+/-0.02 over the whole high-energy tuning range. Experimental results are compared with numerical simulations and a simple model.     

Tunable, high-repetition-rate, femtosecond pulse generation in the ultraviolet

We report efficient generation of tunable femtosecond pulses in the ultraviolet (UV) by intracavity doubling of a visible femtosecond optical parametric oscillator (OPO). The OPO, based on a 400 microm BiB3O6 crystal and pumped at 415 nm in the blue, can provide visible femtosecond signal pulses across 500-710 nm. Using a 500 microm crystal of beta-BaB2O4 internal to the OPO cavity, efficient frequency doubling of the signal pulses into the UV is achieved, providing tunable femtosecond pulses across 250-355 nm with up to 225 mW of average power at 76 MHz. Cross-correlation measurements result in UV pulses with durations down to 132 fs for 180 fs blue pump pulses.     

Quartic-phase-limited grism-based ultrashort pulse shaper

By replacing the dispersive element in a zero-dispersion pulse shaper with a grism, we have constructed a quartic-phase-limited pulse shaper. We demonstrate compensation of 4500 fs2 without the use of a dynamic element in the pulse shaping line, which is approximately the amount of dispersion induced by a typical multiphoton microscope. We also demonstrate that detuning the pulse shaper to compensate for quadratic phase induces negligible spatial chirp, thereby maintaining a high-quality focal spot for a microscopy setup.     

Generation of tunable 7-fs ultraviolet pulses: achromatic phase matching and chirp management

Ultraviolet pulses with a duration of 7 fs are efficiently generated by frequency doubling the output of a noncollinear optical parametric amplifier. The ultraviolet pulses are tunable between 275 to 335 nm. The acceptance bandwidth of the doubling crystal is increased by a factor of 80 through high-order achromatic phase matching. The chirp of the visible pulses and the dispersion introduced along the beam path are compensated partially before and partially after the doubling crystal. For the design of the dispersion management, we investigate the second harmonic generation of pulses with mixed orders of chirp and explicitly discuss the transfer of the spectral phase in frequency doubling. A simple analytic theory is derived that correctly describes frequently observed spectral narrowing effects. We find that chirped SHG avoids spectral narrowing and allows for precompression of dispersion encountered in the ultraviolet beam path. We apply chirped SHG to generate 18.7 fs ultraviolet pulses in an extremely simple setup. PACS 42.65.Re; 42.65.Ky; 42.65.Yj     

Compression of fiber supercontinuum pulses to the Fourier-limit in a high-numerical-aperture focus

A multiphoton intrapulse interference phase scan (MIIPS) adaptively and automatically compensates the combined phase distortion from a fiber supercontinuum source, a spatial light modulator pulse shaper, and a high-NA microscope objective, allowing Fourier-transform-limited compression of the supercontinuum pulses at the focus of the objective. A second-harmonic-generation-based method is employed to independently validate the transform-limited compression. The compressed pulses at the focus of the objective have a tunable duration of 10.8-38.9 fs (FWHM), a central wavelength of ~1020 nm, an average power of 18-70 mW, and a repetition rate of 76 MHz, permitting the application of this source to nonlinear optical microscopy and coherently controlled microspectroscopy.     

Cross-validation of theoretically quantified fiber continuum generation and absolute pulse measurement by MIIPS for a broadband coherently controlled optical source

The predicted spectral phase of a fiber continuum pulsed source rigorously quantified by the scalar generalized nonlinear Schrödinger equation is found to be in excellent agreement with that measured by multiphoton intrapulse interference phase scan (MIIPS) with background subtraction. This cross-validation confirms the absolute pulse measurement by MIIPS and the transform-limited compression of the fiber continuum pulses by the pulse shaper performing the MIIPS measurement, and permits the subsequent coherent control on the fiber continuum pulses by this pulse shaper. The combination of the fiber continuum source with the MIIPS-integrated pulse shaper produces compressed transform-limited 9.6 fs (FWHM) pulses or arbitrarily shaped pulses at a central wavelength of 1020 nm, an average power over 100 mW, and a repetition rate of 76 MHz. In comparison to the 229-fs pump laser pulses that generate the fiber continuum, the compressed pulses reflect a compression ratio of 24.     

Broadband noncollinear optical parametric amplifier using a single crystal

A noncollinear optical parametric amplifier in which blue pump pulse generation as well as parametric amplification takes place in the same nonlinear crystal is presented. Broadband spectra tunable throughout over 100 nm in the visible were generated in a simple setup. The green output pulses were compressed to 65 fs, and efficiency (IR-to-visible) of 1.8% was achieved.     

Pulse-front-matched optical parametric amplification for sub-10-fs pulse generation tunable in the visible and near infrared

A noncollinear optical parametric amplifier is presented that generates transform-limited sub-10-fs pulses that are tunable in both the visible and the near infrared (NIR). The pulse-front-matched pump geometry realizes tilt-free signal amplification, and pulses as short as 6.1 fs can be obtained from 550 to 700 nm. The large angular dispersion of the idler specific to the group-velocity-matching interaction is effectively eliminated by a grating-telescope compensator, and 9-fs NIR pulses are also successfully obtained from 900 to 1300 nm. This is believed to be the first tunable sub-10-fs light source.     

High energy femtosecond OPA pumped by 1030 nm Yb:KGW laser.

We present a traveling-wave-type optical parametric amplifier (OPA) pumped at 1.03 μm by a Yb:KGW laser that produces tunable high-energy pulses of 6.5–4 μJ in the mid-infrared (mid-IR) region from 3.6 to 7 μm. Pumping with negatively chirped pulses generates nearly transform-limited (TL) mid-IR pulses of 300–330 fs length. Pumping with TL pulses of 200 fs not only decreases the output energy by a factor of 1.5, but also decreases the mid-IR pulse-length to 160 fs after additional compression. The compact and simple OPA setup is ideal for femtosecond infrared experiments in the fingerprint region.     

Mid-infrared pulse generation using a sub-picosecond OPO

We describe a synchronously pumped LBO parametric oscillator pumped by a cw mode-locked Ti:Sapphire laser. We demonstrate the synchronization between the pump pulses and the signal pulses by measuring a 100 fs cross correlation. We then generate pulses tunable from 2.6 to 5.3 µm by mixing the signal and idler pulses in an AgGaS₂ crystal and obtain as much as 400 µW of average power.     

Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber

We report on the spectral broadening of ~1 μJ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8%. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.     

Femtosecond OPO based on LBO pumped by a frequency-doubled Yb-fiber laser-amplifier system for CARS spectroscopy

We present detailed investigations of a femtosecond green-pumped optical parametric oscillator (OPO) based on lithium triborate. As pump source, a frequency-doubled Yb-fiber laser-amplifier system is used. The OPO generates signal pulses tunable over a spectral range from 780 to 940 nm and idler pulses tunable from 1630 to 1190 nm. More than 250 mW are generated in the signal beam and more than 300 mW in the idler beam. Without dispersion compensation chirped signal pulses with a pulse duration between 100 and 250 fs are measured. Using this system for coherent anti-Stokes Raman scattering spectroscopy, vibrational resonances between 1110 and 6760 cm⁻¹ can be excited. Due to the chirped pulses, a spectral resolution of 100 cm⁻¹ is achieved, which is 2.5 times higher compared to an excitation with time-bandwidth limited pulses.     

Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter

Femtosecond pulses with center wavelengths between 470 and 750 nm are generated in a single-stage type I BBO optical parametric amplifier pumped by a frequency-doubled 1-kHz Ti:sapphire amplifier. A high-quality white-light continuum is used as the seed. Pulse durations as short as 16 fs and pulse energies of as much as 11 microJ are observed. The quantum efficiency is ~25% for both 7- and 40-microJ pump pulses. This unique combination of ultrashort pulse duration and high conversion is made possible by noncollinear phase matching that permits a sufficiently large amplification bandwidth. Simultaneously the group velocities of the signal and the idler are effectively matched. As a result widely tunable sub-20-fs pulses can be generated in a nonlinear crystal as thick as 2 mm.     

Greater signal, increased depth, and less photobleaching in two-photon microscopy with 10 fs pulses

The fundamental advantages to using ultrafast (?100 fs) laser pulses in two-photon microscopy for biomedical imaging are seldom realized due to chromatic dispersion introduced by the required high numerical aperture microscope objective. Dispersion is eliminated here by using the multiphoton intrapulse interference phase scan (MIIPS) method on pulses with a bandwidth greater than 100 nm full width at half maximum. Higher fluorescence intensity, deeper sample penetration, and improved signal-to-noise ratio are demonstrated quantitatively and qualitatively. Due to the higher signal intensity obtained after MIIPS compensation, lower laser power is required, which decreases photobleaching. The observed advantages are not realized if group delay dispersion is compensated for while higher-order dispersion is not. By using a pulse shaper and taking advantage of the broad spectrum of the ultrafast laser, selective excitation of different cell organelles is achieved due to the difference in nonlinear optical susceptibility of different chromophores without requiring an emission filter wheel. Experiments on biological specimens, such as HeLa cells and mouse kidney tissue samples, illustrate the advantages to using sub-10 fs pulses with MIIPS compensation in the field of two-photon microscopy for biomedical imaging.     

Few-cycle pulse characterization with an acousto-optic pulse shaper

An acousto-optic pulse shaper has been used to characterize few-cycle pulses generated in a hollow-core fiber. A grism pair precompensates for the dispersion of the acousto-optic crystal, allowing the full pulse-shaping window to be used for replica generation rather than self-compensation. A 9.4 fs pulse was measured, the shortest ever measured with an acousto-optic pulse shaper, to our knowledge.