We present a method for implementing optical heterodyne detection using a diffractive optic for time-resolved transient-grating experiments. This technique does not require active phase locking of pulse pairs to achieve interferometric stability. The phase stability, intrinsic time resolution, and signal amplification are demonstrated experimentally through Raman scattering in carbon disulfide.

A chain of four Tm-doped fibers amplified a single-frequency, 2040 nm diode laser to 608 W with M(2)=1.05+/-0.03, limited by available pump power. Stimulated Brillouin scattering limits were investigated by splicing different lengths of passive fiber to the output of the final amplifier stage. Integrated rms phase noise above 1 kHz was less than lambda/30, suggesting the possibility of further scaling via coherent beam combining. To our knowledge, this is the highest power obtained from any single-frequency, single-mode fiber laser.

The initial structural evolution of carboxymyoglobin (MbCO) following photodissociation of CO is studied using optically heterodyne-detected (OHD) transient grating (TG) spectroscopy. This method provides detailed dynamical information on the electronic and structural states of the heme protein following photoexcitation. The phase anisotropy of MbCO is found to develop on subpicosecond to picosecond time scales and is much greater than can be attributed to the symmetry of the heme dipole transition. Control studies of carboxyprotoheme and deoxymyoglobin were used to identify the components due to protein structural relaxation and thermal relaxation, respectively. A geometric decomposition of the MbCO grating signals into contributions relative to the molecular axes provides evidence that the protein effectively changes its shape within 500 fs following ligand dissociation. These anisotropic mass displacements are a signature of functionally important motions since they imply a certain degree of directionality or mode selective coupling to the response. The anisotropic relaxation and observed dynamics provide further evidence that the low-frequency collective modes of proteins play an important role in transducing reaction forces into functions.

Coherent combining efficiency is examined analytically for large arrays of non-ideal lasers combined using filled aperture elements with nonuniform splitting ratios. Perturbative expressions are developed for efficiency loss from combiner splitting ratios, power imbalance, spatial misalignments, beam profile nonuniformities, pointing and wavefront errors, depolarization, and temporal dephasing of array elements. It is shown that coupling efficiency of arrays is driven by non-common spatial aberrations, and that common-path aberrations have no impact on coherent combining efficiency. We derive expressions for misalignment losses of Gaussian beams, providing tolerancing metrics for co-alignment and uniformity of arrays of single-mode fiber lasers.

A three-stage Yb-fiber amplifier emitted 1.43 kW of single-mode power when seeded with a 25 GHz linewidth master oscillator (MO). The amplified output was polarization stabilized and phase locked using active heterodyne phase control. A low-power sample of the output beam was coherently combined to a second fiber amplifier with 90% visibility. The measured combining efficiency agreed with estimated decoherence effects from fiber nonlinearity, linewidth, and phase-locking accuracy. This is the highest-power fiber laser that has been coherently locked using any method that allows brightness scaling.