Carrier-envelope offset stable, coherently combined ytterbium-doped fiber CPA delivering 1 kW of average power.

We present a carrier-envelope offset (CEO) stable ytterbium-doped fiber chirped-pulse amplification system employing the technology of coherent beam combining and delivering more than 1 kW of average power at a pulse repetition rate of 80 MHz. The CEO stability of the system is 220 mrad rms, characterized out-of-loop with an f-to-2f interferometer in a frequency offset range of 10 Hz to 20 MHz. The high-power amplification system boosts the average power of the CEO stable oscillator by five orders of magnitude while increasing the phase noise by only 100 mrad. No evidence of CEO noise deterioration due to coherent beam combining is found. Low-frequency CEO fluctuations at the chirped-pulse amplifier are suppressed by a "slow loop" feedback. To the best of our knowledge, this is the first demonstration of a coherently combined laser system delivering an outstanding average power and high CEO stability at the same time.

Spectral phase noise analysis of a cryogenically cooled Ti:Sapphire amplifier.

The spectral phase noise of a cryogenically cooled Ti:Sapphire amplifier was analyzed by spectrally resolved interferometry. Since a relative phase difference measurement is performed, the effect of the amplifier stage can be determined with high precision. Contributions of the cooling system to the spectral phase noise were found to be below 50 mrad for both the vacuum pumps and the cryogenic system. The carrier-envelope phase noise of thermal and mechanical origin was also determined for different repetition rates of laser operation. Mechanical vibrational spectra were recorded by an accelerometer for different stages of operation and compared to the interferometric phase noise measurements.

Carrier-envelope phase drift measurement of picosecond pulses by an all-linear-optical means.

Carrier-envelope phase (CEP) drift of a pulse train of 2 ps pulses has been measured by a multiple beam interferometer. The round trip time of the interferometer is slightly mistuned from the pulse sequence, leading to spectral interference fringes. We extract the pulse-to-pulse CEP drift from the position of the spectral interference pattern. The length of the interferometer has been actively stabilized to ±10 nm, which sets the ultimate limit on the accuracy of the measurement to 78 mrad, while the CEP-drift (rms) noise of the measurement was 127 mrad (at 800 nm).

Fast neutron generation with few-cycle, relativistic laser pulses at 1 Hz repetition rate.

Laser-driven deuterons generate neutrons with a mean energy of 2.5 MeV, through the 2H(d,n) fusion reaction in a deuterated polyethylene (dPE) tablet. The deuterium ions are accelerated by 12 fs, 21 mJ laser pulses interacting with a 0.2 µm thin dPE foil at a peak intensity of 1018 W/cm2. The laser was operated at 1 Hz repetition rate in bursts of 75 shots. The interaction was characterized and recorded for each laser shot. The ion spectra were measured in the forward and backward directions by Thomson ion spectrometers. Neutron events were detected by a time-of-flight (ToF) system consisting of four plastic scintillators positioned at various angles around the experimental chamber. The maximum cut-off energy of the forward accelerated protons and deuterons was close to 1.4 MeV and 1 MeV, while the mean values are 428 ± 63 keV and 433 ± 80 keV, respectively. Analysis of ToF distributions from 3128 shots resulted in an average yield of 1142 ± 59 neutrons per shot in the energy range of 1.5-4 MeV. The energy distribution of forward-directed neutrons peaks between 3 and 3.5 MeV. Angular dependence analysis showed a perpendicular minimum and a maximum along the deuteron beam, consistent with the expected distribution from the literature and our simulation results.

Two-dimensional single-shot measurement of angular dispersion for compressor alignment.

Misalignment of the stretcher-compressor stages of chirped pulse amplification (CPA) lasers can aggravate the spatiotemporal shape of ultrashort pulses. We demonstrate a simple technique for angular dispersion measurement, which offers real-time single-shot two-dimensional characterization across the entire beam profile. The accuracy of our pilot experiment approaches its current theoretical limit of 0.1 µrad/nm. Unlike the current one-dimensional techniques working in the near field, the method works in the far field; hence, it is especially appropriate for assisting the most accurate alignment of a CPA laser compressor ensuring the maximum intensity on the target.

External cavity enhancement of picosecond pulses with 28,000 cavity finesse.

We report on the first demonstration, to the best of our knowledge, of the locking of a Fabry-Perot cavity with a finesse of 28,000 in the pulsed regime. The system is based on a stable picosecond oscillator, an ultrastable cavity with high-reflection mirrors, and an all-numerical feedback system that allows efficient and independent control of the repetition rate and the pulse to pulse carrier-to-envelop phase drift (CEP). We show that the carrier to envelop phase can have a dramatic effect even for pulses with hundreds of cycles. Moreover, we have succeeded in unambiguously measuring the CEP of a 2 ps pulse train. Finally, we discuss the potential of our findings to reach the MW average power level stored in an external cavity enhancement architecture.

Agile linear interferometric method for carrier-envelope phase drift measurement.

A bandwidth-independent and linear interferometric method for the measurement of the carrier-envelope phase drift of ultrashort pulse trains is demonstrated. The pulses are temporally overlapped in a resonant multiple-beam interferometer. From the position of the spectral interference pattern, the relative carrier-envelope phase between two subsequent oscillator pulses is obtained at data acquisition rates up to 200 Hz. Cross calibration has been performed by f-to-2f interferometry in two independent experiments. The optical length of the interferometer has been actively stabilized, leading to a phase jitter of 117 mrad (rms). These results indicate a reduced noise and quicker data acquisition in comparison with previous linear methods for measuring the carrier-envelope phase drift.

Measurement of pressure dependent nonlinear refractive index of inert gases.

The propagation of high intensity laser beams is excessively affected by optical nonlinear effects, thereby the knowledge of the nonlinear refractive indices of the beam guiding media is indispensable in the design of laser systems and experiments. Apart from undesired self-focusing, several areas of modern laser spectroscopy can utilize optical nonlinearity, from LiDAR measurements to filamentation. In this paper we report on a direct measurement of pressure dependent nonlinear refractive index of Ar, N2, Ne, Xe, and air between 0.05 mbar and 1 bar, based on the powerful technique called spectrally and spatially resolved interferometry. In this way the total value of nonlinear refractive index is measured, that is the sum of all elementary phenomena contributing to the intensity dependent refractivity of the gases.

Preservation of the carrier envelope phase during cross-polarized wave generation.

The preservation of carrier envelope phase (CEP) during Cross-Polarized Wave Generation (XPWG) is demonstrated through two independent experiments based on the spatially and spectrally resolved interference fringes formed by the XPW beam and its fundamental. In a first measurement, we found that the vertical fringe position on the spatial detector was maintained over many consecutive laser shots, implying practically no change in relative CEP between the XPW and the fundamental. In a second experiment, we measured the change in relative CEP between the XPW and fundamental beam by systematically varying the amount of material dispersion inside the XPW arm of the interferometer. The recorded rate of relative phase change was in excellent agreement with the theoretical value.

Dispersion measurement of inert gases and gas mixtures at 800 nm.

Dispersion of femtosecond laser pulses propagating in Ar, He, Kr, N(2), Ne, Xe, and their mixtures is measured by spectrally and spatially resolved interferometry. By varying the gas pressure in a 4.5 m long tube between 0.05 mbar and ambient pressure, the first, second, and third order phase derivatives of broadband laser pulses are determined at 800 nm under standard conditions. The dispersion of gases and gas mixtures obeys the Lorentz-Lorenz formula with an accuracy of 0.7%. Based on the measured pressure dependent dispersion values in the near infrared and the refractive indices available from the literature for the ultraviolet and visible, a pressure dependent Sellmeier-type formula is fitted for each gas. These common form, two-term dispersion equations provide an accuracy between 4.1x10(-9) (Ne) and 4.3x10(-7) (Xe) for the refractive indices, from UV to near IR.