For powers exceeding a sharp threshold in the vicinity of several hundred watts the beam quality from some narrow bandwidth fiber amplifiers is severely degraded. We show that this can be caused by transverse thermal gradients induced by the amplification process.

We have measured asymmetric photoelectron angular distributions for atomic rubidium. Ionization is induced by a one-photon interaction with 280 nm light and by a two-photon interaction with 560 nm light. Interference between the even-and odd-parity free-electron wave functions allows us to control the direction of maximum electron flux by varying the relative phase of the two laser fields. PACS numbers: 32.80.Fb, 32.80.Rm Interferences between different optical interactions involving the same initial and final states have attracted a great deal of attention lately. We showed previously [1] the variation of atomic excitation probability in mercury with the relative phase of the fields when a laser field and its third harmonic were focused into a mercury vapor cell. The interfering interactions have also been demonstrated in the total ionization rate in a molecular system, HC1, by Park, Lu, and Gordon [2]. We have also demonstrated the effect of phase and amplitude variations of focused beams on the interference measurements [3]. Muller et al. [4] exploited the interfering interactions as a probe of above threshold ionization (ATI) effects in atomic krypton using an electron detector sensitive to electrons ejected only in the direction of the laser polarization. Calculations of this interference effect for high-intensity fields have been reported [5,6]. Similar results involving oneand two-photon absorption by a photocathode were reported by Baranova et al. [7,8]. These authors have recently extended their technique to an atomic system [9], and report interference for two-photon versus one-photon ionization of the 4s state of atomic sodium. Secondharmonic generation in optical fibers has been attributed [10] to the asymmetry reported in this Letter as well.In this Letter we will discuss our observations of asymmetric photoelectron angular distributions in rubidium resulting from this type of interference. To induce this asymmetry, we generate a laser field consisting of two frequencies, one an ultraviolet field capable of photoionizing the atom through the absorption of a single photon, and the second a visible field for which the absorption of two photons is required for photoionization. The frequency of the first field is precisely twice that of the second. The continuum state produced upon interaction of the atom with this field can be expressed as a coherent combination of even-parity states (eS and sD) and an oddparity state (eP), and is, therefore, neither symmetric nor antisymmetric. In effect, it is the asymmetry of the field which leads to an asymmetric photoelectron angular distribution. Varying the relative phase and amplitude of the two field components changes the observed asymmetry of the photoelectron angular distribution. Our results clearly show that the photoelectron fluxes in opposite directions are anticorrelated. The magnitude of the asymmetry is as large as 4:1. In this Letter, we will first present a simple theory of the asymmetry based on perturbation theory, then discuss our experiment...

We show by numerical modeling that saturation of the population inversion reduces the stimulated thermal Rayleigh gain relative to the laser gain in large mode area fiber amplifiers. We show how to exploit this effect to raise mode instability thresholds by a substantial factor. We also demonstrate that when suppression of stimulated Brillouin scattering and the population saturation effect are both taken into account, counter-pumped amplifiers have higher mode instability thresholds than co-pumped amplifiers for fully Yb3+ doped cores, and confined doping can further raise the thresholds.

We measured bulk and surface dielectric breakdown thresholds of pure silica for 14 ps and 8 ns pulses of 1064 nm light. The thresholds are sharp and reproducible. For the 8 ns pulses the bulk threshold irradiance is 4:75 AE 0:25 kW=μm 2. The threshold is approximately three times higher for 14 ps pulses. For 8 ns pulses the input surface damage threshold can be made equal to the bulk threshold by applying an alumina or silica surface polish.

Using numerical simulations of thermally induced mode coupling we show how the instability threshold can be substantially reduced if the pump or injected signal is modulated in the kHz range. We also show how the mode coupling gain varies with the frequency offset of the parasitic mode. We model thresholds when the source of detuned light is quantum background, amplitude modulation of the pump power, and amplitude modulation of the signal seed. We suggest several key experimental and modeling tests of our model.