Abstract : We demonstrate an Yb-doped polarizing all-solid photonic bandgap fiber for single-polarization and single-mode operation with an effective mode area of 1150m2, a record for all-solid photonic bandgap fibers. The differential polarization mode loss is measured to be 5dB/m over the entire transmission band with a 160nm bandwidth and 15dB/m on the short wavelength edge of the band. A 2.6m long fiber was tested in a laser configuration producing a linearly polarized laser output with a PER value of 21dB without any polarizer, the highest for any fiber lasers based on polarizing fibers.
Pulses from two mode-locked lasers have been synchronized to better than 2 ps. The technique, similar to that used by Halbout and Grischkowsky 1 uses the output of a passively mode-locked CPM dye laser to generate a radiofrequency (rf) signal which can be used to drive a mode-locker for a second laser. In our case, the second laser is a diode pumped Nd:YLF (DPY) FM mode-locked with a lithium-niobate phase modulator. 2 FM mode-locking has two advantages over AM mode-locking: it does not require a rf divide by two element, and the phase modulator tank circuit can be easily tuned without reducing its Q. The tank circuit has a relatively low Q that allows the DPY laser to be adjusted to the CPM laser frequency without tuning the circuit. The CPM can be set optimally to produce the shortest pulses, and the DPY adjusted to operate in synchrony. The DPY generates pulses as short as 15 ps at 100 Mhz; it operates stably in two modes corresponding to pulse durations of 15 and 30 ps. The shortest pulses, measured by a cross correlation with pulses from the CPM, are neither Gaussian nor hyperbolic secant in shape. They have an asymmetric shape with one or two satellite pulses preceding or following the main pulse depending on the cavity length detuning.
The characterization of atmospheric winds on a global basis is a key parameter required for accurate weather prediction. The use of a space based lidar system for remote measurement of wind speed would provide detailed and highly accurate data for future weather prediction models. This paper reports the demonstration of efficient third harmonic conversion of a 1 micrometer laser to provide an ultraviolet (UV) source suitable for a wind lidar system based on atmospheric molecular scattering. Although infrared based lidars using aerosol scattering have been demonstrated to provide accurate wind measurement, a UV based system using molecular or Rayleigh scattering will provide accurate global wind measurements, even in those areas of the atmosphere where the aerosol density is too low to yield good infrared backscatter signals. The overall objective of this work is to demonstrate the maturity of the laser technology and its suitability for a near term flight aboard the space shuttle. The laser source is based on diode-pumped solid-state laser technology which has been extensively demonstrated at TRW in a variety of programs and internal development efforts. The pump laser used for the third harmonic demonstration is a breadboard system, designated the Laser for Risk Reduction Experiments (LARRE), which has been operating regularly for over 5 years. The laser technology has been further refined in an engineering model designated as the Compact Advanced Pulsed Solid-State Laser (CAPSSL), in which the laser head was packaged into an 8 x 8 x 18 inch volume with a weight of approximately 61 pounds. The CAPSSL system is a ruggedized configuration suitable for typical military applications. The LARRE and CAPSSL systems are based on Nd:YAG with an output wavelength of 1064 nm. The current work proves the viability of converting the Nd:YAG fundamental to the third harmonic wavelength at 355 nm for use in a direct detection wind lidar based on atmospheric Rayleigh scattering.
We report the effectiveness of multiple-cladding-resonance photonic bandgap fiber for suppressing mode instability by demonstrating pump-limited single-mode output power of ~1kW in a 60µm-core fiber, a record for any fiber lasers at this core diameter.
Optical Kerr effect (OKE) studies of myoglobin and water at wavelength of 627 nanometers with 45 femtosecond pulses have been performed. The nonresonant response of water consists of electronic, translational, librational and relaxational components. The low frequency components observed in the water OKE study are in qualitative agreement with previous depolarized light scattering (DLS) studies and molecular dynamics (MD) simulations. Operation on the edge of the Q-band of the myoglobin resulted in the pulse broadening to 100 fs. Two relaxational components, 195 fs and 2.4 +/- .2 ps were observed in both the carboxy and deoxy myoglobin samples studied. The 195 fs component is assigned to the OKE response of water while the 2.4 ps component is related to the transient birefringence of the myoglobin. A discussion of the origin of the transient signal as well as the calculation leading to the assignment to birefringence as opposed to dichroism is included. With this interpretation, the observed dynamics are related to the low frequency modes of the protein. Information on these modes is needed to understand the initial events that direct functionally important structural changes.
A diode-pumped Yb:YAG laser with a novel end-pumped zigzag slab architecture has been developed. This architecture provides uniform transverse pump profiles, conduction cooling of the laser crystal, mechanical robustness, and ready scalability to higher powers. At room temperature the laser emits 415 W of cw power with 30% optical conversion efficiency. An image-inverting stable resonator permits a high-brightness output of 252 W with linear polarization and an average M(2) beam quality of 1.45. Q-switched pulse energies of as much as 20 mJ and average Q-switched powers of as much as 150 W were obtained while M(2) was maintained at <1.5.
Two tapered fiber amplifier chains were coherently combined to generate 0.42-mJ, 1-ns pulses with 79% efficiency despite 38 radians of intra-pulse phase distortion. A recursive real-time chirp control method significantly reduced these phase errors.
The method used to deduce the spectral density distribution of intermolecular and intramolecular (vibrational) degrees of freedom in the liquid state from optical heterodyne detected optical (Raman-induced) Kerr effect (OHD-RIKE) measurements is reexamined within a multimode Brownian oscillator model. The ramifications of nonlinear coupling of the nuclear degrees of freedom to the medium polarizability are explored for discrimination between "homogeneous" and "inhomogeneous" contributions to the vibrational spectral density. Under physically reasonable assumptions, an estimation of the homogeneous contribution to the vibrational line shape can be made from the OHD-RIKE observable (if nonlinear coupling is nonnegligible). The model is developed generally, and calculations are applied specifically to temperature-dependent OHD-RIKE measurements of liquid water. The results indicate that the line broadening in the low-frequency vibrational distribution due to the hydrogen-bonded network structure of liquid water is mostly inhomogeneous, with an effective homogeneous relaxation time of 350 fs at 24 °C.
