Toward single-cycle laser systems

Few-cycle pulse generation based on Ti:sapphire, Cr:forsterite, and Cr:YAG gain media is reviewed. The dynamics of these laser systems is well understood in terms of soliton and dispersion managed soliton formation stabilized by artificial saturable absorber action provided by Kerr-lens modelocking. These systems generate 5-, 14-, and 20-fs pulses with spectral coverages of 600-1150, 1100-1600, and 1200-1500 nm, respectively. The design of dispersion compensating laser optics providing high reflectivity and prismless operation over this bandwidth is discussed. A novel active synchronization scheme based on balanced optical cross correlation, the equivalent to balanced microwave detection, for synchronization of independently mode-locked lasers is introduced. Its use in synchronizing an octave-spanning Ti:sapphire laser and a 30-fs Cr:forsterite laser yields 300 attoseconds timing jitter measured from 10 mHz to 2.3 MHz. The spectral overlap between the two lasers is large enough to enable direct detection of the difference in the carrier-envelope offset frequency between the two lasers. These are the most important steps in the synthesis of single-cycle optical pulses with spectra spanning 600-1600 nm.     

High-dynamic-range laser amplitude and phase noise measurementtechniques

We describe techniques for making sensitive and high-dynamic-range measurements of laser amplitude and envelope phase noise (timing jitter) in the frequency domain at the shot-noise limit. Examples of amplitude noise measurements on continuous-wave argon-ion and diode-pumped solid-state lasers used for pumping a femtosecond Ti:sapphire laser are presented. Amplitude and phase noise measurements for the Ti:sapphire laser are also presented, showing correlation between pump laser amplitude modulation (AM) spectra and the resulting AM and phase noise. Characteristics of the measurement system components are discussed, along with examples of the impact these have on achieving reliable high-dynamic-range measurement capability     

Ultrafast, ultrahigh-peak, and high-average power Ti:sapphire laser system and its applications

We review progress in the generation of multiterawatt optical pulses in the 10-fs range. We describe a design, performance, and characterization of a Ti:sapphire laser system based on chirped-pulse amplification, which has produced a peak power in excess of 100 TW with sub-20-fs pulse durations and an average power of 19 W at a 10-Hz repetition rate. We also discuss extension of this system to the petawatt power level and potential applications in the relativistic, ultrahigh intensity regimes.     

Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd/sup 3+/ picosecond pump laser from a single broadband Ti:Sapphire oscillator

We demonstrate direct simultaneous seeding of a few-cycle optical parametric chirped pulse amplifier (OPCPA) in the 700-1000-nm spectral range, and of a Nd:YLF amplifier emitting 30-ps pulses at 1053 nm by use of a chirped-mirror 6-fs Ti:sapphire oscillator. This approach of employing a single master oscillator to drive two power amplifiers simplifies the pump laser design and is applied to eliminate the timing jitter between the seed and the pump pulses in the OPCPA chain. We show that 10 mJ fundamental picosecond pump pulses with the intensity contrast in excess of 10/sup 4/ relative to the nanosecond Q-switched background can be achieved with the seed intensity available in the edge of the oscillator spectrum around 1053 nm. Cross-correlation measurements between the picosecond pump and femtosecond oscillator pulses reveal no traceable timing jitter between the OPCPA pump and seed pulses. The estimated long-term jitter of 0.3 ps is attributed to the thermal expansion of the cavity of the Nd:YLF regenerative amplifier.     

Frequency conversion of Ti:sapphire-based femtosecond laser systemsto the 200-nm spectral region using nonlinear optical crystals

We review the linear and nonlinear optical properties of crystals transparent near and below 200 nm and suitable for up conversion of femtosecond Ti:sapphire laser sources, β-BaB₂O₄ , the crystal with the largest birefringence of all presently available materials, is investigated experimentally as a quadrupler by mixing the fundamental and the third harmonic both using a 1-kHz repetition rate Ti:sapphire regenerative amplifier and a 82-MHz mode-locked Ti:sapphire laser. Milliwatt average powers near 200 nm are achieved in both cases. The sub-200-fs pulses at the fourth harmonic are almost bandwidth limited. Sum-frequency generation as a method for upconversion of femtosecond pulses is experimentally studied by mixing the fourth harmonic generated down to 189 nm by the regenerative amplifier with a parametrically generated femtosecond pulse in the infrared. Pulse energies at the microjoule level are produced with LiB ₃O₅ above 180 nm. Li₂B₄O₇ shows superior performance in the 170-180-nm range, and the shortest wavelength achieved with KB₅ O₈·4H₂O is 166 nm     

Design and construction of a TW-class 12-fs Ti:Sapphire chirped-pulse amplification system

We describe a design and a construction of a TW-class 12-fs Ti:sapphire chirped-pulse amplification system. We developed a broadband pulse stretcher, a broadband gain-narrowing compensator, broadband high-energy mirrors, high-energy dichroic chirped mirrors, a dispersion compensator, and a broadband pulse compressor for /spl sim/10-fs pulse generation. Utilizing these optical devices, we demonstrated a generation of 12-fs pulses from a 10-Hz-repetition-rate Ti:sapphire chirped-pulse multipass amplifier system and a 1-kHz-repetition-rate Ti:sapphire chirped-pulse regenerative amplifier system. Optimized designs of broadband Ti:sapphire amplifiers with multilayer gain-narrowing compensators and an adaptive dispersion compensator with a spatial light modulator contribute to the shorter pulse amplification.     

Reconstruction of the time profile of femtosecond laser pulsesthrough cross-phase modulation

Experimental results concerning the determination of the time profile of femtosecond laser pulses are presented. The method is based on the analysis of the changes of the laser spectrum induced at different delays by cross-phase modulation. Pulses from a 30-fs Ti:sapphire laser at 800 nm are characterized over a high dynamic range. Second-harmonic pulses at 400 nm from the same laser are also characterized, exhibiting the expected improvement in contrast ratio     

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated ≈30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses     

Phase-controlled amplification of few-cycle laser pulses

Intense ultrashort waveforms of light that can be produced with an exactly predetermined electromagnetic field are essential in a number of applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires stabilization of the carrier-envelope phase. The authors analyze different schemes of phase-stable pulse amplification and identify constraints limiting the precision with which the phase can be maintained. Next, they describe a phase-stabilized laser system based on a 20-fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing pulses in the few-cycle regime. It is shown that the amplifier introduces only a slow millihertz phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. This residual phase drift is assigned primarily to the beam pointing instability and can be precompensated in the phase-control loop of the seed oscillator using a feedback signal from a phase detector placed in the amplifier output. The phase stability of the resultant 5-fs 400-/spl mu/J pulses at a 1-kHz repetition rate is subsequently independently verified by higher order harmonic generation, in which different carrier-envelope phase settings are shown, both theoretically and experimentally, to produce distinctly different spectral shapes of the XUV radiation. From a series of such spectral patterns, the authors succeed in calibrating the value of the carrier envelope phase (with a /spl plusmn//spl pi/ ambiguity), which in turn allows them to fully characterize the temporal structure of the electric field of the laser pulses. The estimated precision of the phase control on the XUV target is better than /spl pi//5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to /spl sim/ 250 as and opens the way to generate stable isolated attosecond pulses.     

Progress on collisionally pumped optical-field-ionization soft X-ray lasers

We present the status of optical field ionization soft X-ray lasers. The amplifying medium is generated by focusing a high-energy circularly polarized 30-fs 10-Hz Ti: sapphire laser system in a gaseous medium. Using xenon or krypton, strong laser emission at 41.8 and 32.8 nm, respectively, has been observed. After presenting the basis of the physics, we present recent characterization of the sources as well as dramatic improvement of their performances using the waveguiding technique.     

Parametric amplification and self-compression of ultrashort tunablepulses

Using a 40-fs Ti:sapphire pump laser, we produced 20-fs infrared pulses in a β-barium borate (BaB₂O₄) optical parametric amplifier without any external dispersion compensation. These results are in good agreement with a novel semi-analytical computer simulation. We also demonstrate that cascading of second-order nonlinear effects yields simultaneous production of ultrashort visible pulses from a near-infrared (NIR) pump     

InGaAs Quantum-Dot Mode-Locked Laser Diodes

This paper presents a selection of recent advances on two-section passively mode-locked InGaAs-based quantum-dot laser diodes. Pulse generation is demonstrated for repetition rates ranging from 310 MHz to 240 GHz, and with pulse durations ranging from the picosecond to the sub-400 fs regime. Mode-locking trends in these devices are discussed, and device performance improvements in terms of pulse duration, output power, and noise properties are presented. Design rules for reducing the pulse duration, increasing the output power, and improving noise performance are outlined. Implementation of tapered waveguide structures yields significant performance improvements, allowing the simultaneous achievement of ultrashort, Fourier-limited pulse generation with low amplitude noise, low timing jitter, and narrow RF linewidths.      

Carrier-envelope offset phase-locking with attosecond timing jitter

Inside a femtosecond laser oscillator, no coupling mechanism between the propagation speeds of the carrier and the pulse envelope exists. Therefore, the relative delay between carrier and envelope of a femtosecond oscillator will exhibit irregular fluctuations unless this jitter is actively suppressed. Both intensity and beam pointing fluctuations in the laser can introduce carrier-envelope phase changes. Based on our analysis, we are capable of reducing or avoiding certain mechanisms by proper design of the laser cavity. We use such an optimized cavity to stabilize the carrier envelope-phase to an external reference oscillator with a long-term residual jitter corresponding to only 10 attoseconds in a (100 kHz-0.01 Hz) bandwidth. This is the smallest long-term timing jitter of a femtosecond laser oscillator demonstrated to date. However, it is important to note that this stabilization was obtained with an f-to-2f heterodyne technique using additional external spectral broadening in a microstructure fiber which introduces additional carrier-envelope phase noise. We present a direct heterodyne measurement of this additional carrier-envelope phase noise due to the continuum generation process.     

Sub-5-fs transform-limited visible pulse source and its applicationto real-time spectroscopy

Transform-limited (TL) visible pulses with as short a duration as 4.7 fs with a 5-μJ pulse energy have been generated for the first time from a novel noncollinear optical parametric amplifier. Both signal-idler group-velocity matching and pulse-front matching are essential to generate coherent down-converted pulses compressible to the TL pulse with more than 150 THz bandwidth. Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near-infrared (900-1300 nm) ranges, achieved by increasing the seed chirp, is also a marked property. Applications for ultrafast spectroscopy of the sub-5-fs pulse source to the real-time spectroscopy of conjugated polymer, a dye molecule, and J-aggregates are also described     

Dispersion control over 150 THz with chirped dielectric mirrors

Ultrabroad-band chirped multilayer dielectric mirrors providing nearly constant negative group delay dispersion over the wavelength range of 640-950 nm and high reflectance between 590 and 970 nm are demonstrated. A key to this performance has been an improved design method, which also substantially reduces the computing time needed for ultimate optimization. The presented devices constitute an enabling technology for producing high-quality terawatt pulses in the sub-10-fs regime. The generation of 5-fs 0.1-TW pulses by using exclusively these mirrors as negative delay line demonstrates this potential     

Self-starting 6.5-fs pulses from a Ti:sapphire laser using asemiconductor saturable absorber and double chirped mirrors

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens-mode-locked Ti:sapphire laser with an average output power of 200 mW at a pulse repetition rate of 86 MHz. We have achieved a mode-locking buildup time of only 60 μs, using a broad-band semiconductor saturable absorber mirror to initiate the pulse formation. The dispersion has been compensated with a prism pair in combination with improved double-chirped mirrors. The prism pair allows for the flexible adjustment of both the duration and the center wavelength of the pulse. The double-chirped mirrors show a high reflectivity better than 99.8% over the full bandwidth of 300 nm and a controlled group delay over more than 250 nm. The choice of a proper output coupler turns out to be critical for ultrashort pulse generation directly from the laser     

Highly phase stable mode-locked lasers

The authors report on stabilizing the carrier-envelope phase of mode-locked Ti:sapphire lasers. Optimization of the construction of the lasers for ease of phase stabilization is discussed. Results demonstrating long-term phase coherence of the generated pulse train are presented, yielding a phase coherence time of at least 326 s, measurement time limited. The conversion of amplitude noise to phase noise in the microstructured fiber, which is used to obtain an octave spanning spectrum, is measured. The resulting phase noise is found to be sufficiently small so as to not corrupt the phase stabilization. Shift of carrier-envelope phase external to the laser cavity due to propagation through a dispersive material is measured.     

Noise of mode-locked semiconductor lasers

Analytical expressions for the amplitude, frequency, timing, and carrier phase noise of mode-locked laser diodes (MLLDs) are derived. It is found both experimentally and theoretically that carrier dynamics contribute to the total noise of MLLDs. In addition, we demonstrate how to capture the high-frequency timing jitter with optical cross correlations     

Subharmonic hybrid mode-locking of a monolithic semiconductor laser

Performance of a subharmonically hybrid mode-locked (SH-ML) monolithic semiconductor laser is investigated. A 33-GHz passively mode-locked distributed Bragg reflector semiconductor laser is stabilized by the injection of an electrical signal with a subharmonic frequency of the laser cavity resonance. Systematic measurements on the phase noise, timing jitter, amplitude modulation, and locking bandwidth are performed for the second- and third-order SH-ML conditions, and the results are compared with the fundamental hybrid mode-locking (FH-ML) case. Low timing jitter of less than 0.6 ps, comparable to that under the FH-ML case, is achieved for the both SH-ML cases. The amplitude modulation imposed by the subharmonic driving frequencies is found to be very small (<-24 dBc) for the second-order SH-ML because of the sufficiently low modulation response of the laser at 16.5 GHz. The third-order SH-ML is found to exhibit a very unique locking characteristics, leading to a maximum locking bandwidth of 56 MHz that is even larger than that for the FH-ML case     

All-Fiber Low-Noise High-Power Femtosecond Yb-Fiber Amplifier System Seeded by an All-Normal Dispersion Fiber Oscillator

We report an all-fiber, high-power, low-noise amplifier system seeded by an all-normal-dispersion-mode-locked Yb-doped fiber laser oscillator. Up to 10.6 W of average power is obtained at a repetition rate of 43 MHz with diffraction-limited beam quality. Amplified pulses are dechirped to sub-160-fs duration in a grating compressor. It is to our knowledge the first high-power source of femtosecond pulses with completely fiber-integrated amplification comprising commercially available components. Long-term stability is excellent. Short-term stability is characterized and an integrated laser intensity noise of $≪$0.2% is reported. We also conclude that all-normal dispersion fiber oscillators are low-noise sources, suitable as seed for fiber amplifiers. Detailed numerical modeling of both pulse generation in the oscillator and propagation in the amplifier provide very good agreement with the experiments and allow us to identify its limitations.