Safe delivery of optical power from space.

More than a billion gigawatts of sunlight pass through the area extending from Earth out to geostationary orbit. A small fraction of this clean renewable power appears more than adequate to satisfy the projected needs of Earth, and of human exploration and development of space far into the future. Recent studies suggest safe and efficient access to this power can be achieved within 10 to 40 years. Light, enhanced in spatial and temporal coherence, as compared to natural sunlight, offers a means, and probably the only practical means, of usefully transmitting this power to Earth. We describe safety standards for satellite constellations and Earth based sites designed, respectively, to transmit, and receive this power. The spectral properties, number of satellites, and angle subtended at Earth that are required for safe delivery are identified and discussed.

Multi-turn all-reflective optical gyroscope.

We use calculation and simulation to characterize an all-reflective monolithic gyroscopic structure that supports 3 sets of orthogonal, spatially dense and continuous helical optical paths. This gyroscope differs from current fiber optic and ring laser gyroscopes primarily in the free space multi-turn nature of the optical path. The design also creates opportunities for introducing gain while minimizing spontaneous emission noise from those gain regions. The achievable angular measurement precision for each axis, given ideal components and no gain, is calculated to be ~0.001 degrees /hr for a structure of ~6.5 cm diameter, ~1 watt average optical power, and a wavelength of 0.5 microm. For fixed power, the uncertainty scales as the reciprocal cube of the diameter of the structure. While the fabrication and implementation requirements are challenging, the needed reflectivities and optical surface quality have been demonstrated in more conventional optics. In particular, the low mass, compact character, and all reflective optics offer advantages for applications in space.

Full cycle, low loss, low distortion phase modulation from multilayered dielectric stacks with terahertz optical bandwidth.

We present a customized multilayered dielectric stack employed as a broadband phase modulator with 6.3 THz optical bandwidth. The bandpass modulator provides up to a full-cycle of near-uniform phase modulation across a defined signal spectrum with maximized transmission and minimized pulse phase distortion. The modulator offers a compact, lightweight approach to active wavefront phase control for large optical apertures without the use of mechanical actuators. The modulator also provides for rapid signal switching. We contrast the narrowband transmission of a standard Distributed Bragg Reflector (DBR) with the broadband transmission of our optimized bandpass modulator. We explore techniques for implementing rapid phase modulation while maintaining high average signal transmission levels.

Simultaneous measurement of group delay and transmission of a one-dimensional photonic crystal.

We characterize both the group delay and the transmission of a layered semiconductor structure in a single easily interpreted plot. The data spans a 50 nm wide spectral range with 1.7 nanometer wavelength resolution, and a 1.3 picosecond wide temporal range with temporal resolution of tens of femtoseconds. Specific data for a 28 period GaAs/AlAs layered photonic band-gap structure that characterizes both group delay and transmission of multiple photonic resonances in a single display are presented and compared to theory.

Optical amplifier for space applications.

We describe an optical amplifier designed to amplify a spatially sampled component of an optical wavefront to kilowatt average power. The goal is means for implementing a strategy of spatially segmenting a large aperture wavefront, amplifying the individual segments, maintaining the phase coherence of the segments by active means, and imaging the resultant amplified coherent field. Applications of interest are the transmission of space solar power over multi-megameter distances, as to distant spacecraft, or to remote sites with no preexisting power grid.

Soliton-repulsion logic gate.

We present numerical simulations of a logic gate based on repulsive interaction in a dual-core fiber and call it a soliton-repulsion logic gate (SRLG). The operation of the SRLG is compared with that of the conventional soliton-dragging logic gate based on cross-phase modulation. The length of the SRLG can be reduced by a factor of 3 over that of the conventional logic gate. By optimizing parameters, terahertz operation can be obtained.

Kerr-lens-mediated dynamics of two nonlinearly coupled mode-locked laser oscillators.

The dynamics of two nonlinearly coupled femtosecond oscillators are investigated for the case where two distinct nonlinear mechanisms are balanced to determine the temporal relationship and properties of the pulses in the two oscillators. In the time domain the shared bleaching of a common absorber creates an attractive mechanism for the pulses, while interactive Kerr lens deflections create a repulsive mechanism. The interplay of these two mechanisms causes a variety of dynamical behaviors, including pulse synchronization, pulse duration switching, and a latching type of amplitude bistability.

Derivation and measurement of the reversible temporal lengthening of femtosecond optical pulses for the case of a four-prism sequence.

An expression is derived for the reversible lengthening of optical pulse durations from the order of 50 fsec to the order of 500 fsec by a four-prism sequence and focusing elements. This temporal lengthening is caused by spatial dispersion of the different frequency components transverse to the direction of propagation at the symmetry plane of the prism sequence. Experimental measurements of the pulse lengthening agree well with calculated values.

Amplification of femtosecond optical pulses using a double confocal resonator.

We experimentally examine a class of multipass amplifiers based on an open double confocal resonator. A preamplifier provides a gain of 800. An intermediate-stage amplifier produces pulse energies of 2 microJ for only 1.4 W of pump power at a repetition rate of 10 kHz. A third structure, which includes a four-prism sequence, provides on each pass adjustable forms of the four basic pulse-shaping mechanisms of saturable gain, saturable absorption, group-velocity dispersion, and self-phase modulation, as well as spectral filtering. This latter structure also reversibly lengthens the pulse duration in the gain medium from 50 fsec to over 450 fsec.

Compression of optical pulses to six femtoseconds by using cubic phase compensation.

We demonstrate that a combination of prisms and diffraction gratings can provide not only quadratic but also cubic phase compensation of ultrashort optical pulses. We obtain compressed pulses as short as 6 fsec.

Optical frequency filter for ultrashort pulses.

We describe an optical filter based on prisms, which provides both spectral filtering and an adjustable correction to group-velocity dispersion. The low losses permit incorporation in a laser oscillator, and the group-velocity correction of the prism sequence has been used with pulses as short as 10 fsec.

Generation of optical pulses as short as 27 femtoseconds directly from a laser balancing self-phase modulation, group-velocity dispersion, saturable absorption, and saturable gain.

We describe an ultrasbort-pulse laser that, under specific operating conditions, balances the mechanisms of conventional passive mode locking and solitonlike pulse shaping in a single resonator to generate optical pulses that are to our knowledge the shortest yet emitted directly from a laser, 27 fsec.

Negative dispersion using pairs of prisms.

We show that pairs of prisms can have negative group-velocity dispersion in the absence of any negative material dispersion. A prism arrangement is described that limits losses to Brewster-surface reflections, avoids transverse displacement of the temporally dispersed rays, permits continuous adjustment of the dispersion through zero, and yields a transmitted beam collinear with the incident beam.

Optical resonator with negative dispersion.

Analysis of an optical ring resonator consisting of a prism and two mirrors demonstrates that such a resonator can have adjustable dispersion of either sign. The dispersion is proportional to the second derivative of the optical path length in the resonator with respect to wavelength. Adjustable dispersion may have important application to the production of ultrashort laser pulses.

Theory of passively mode-locked lasers including self-phase modulation and group-velocity dispersion.

Closed-form analytical solutions are obtained for a passively mode-locked laser for the case in which self-phase modulation and group-velocity dispersion, in addition to the more conventional mechanisms of saturable absorption and gain, shape the laser pulses. Provided that the self-phase modulation and group-velocity dispersion are related in a manner similar to that which causes soliton formation in optical fibers, this additional pulse shaping can reduce the pulse duration below the limit otherwise set by the laser bandwidth.

Amplified femtosecond optical pulses and continuum generation at 5-kHz repetition rate.

Pulses of 90-fsec duration from a cavity-dumped colliding-pulse mode-locked laser have been amplified to microjoule energies at 5-kHz repetition rate using a copper-vapor-laser pump source. Near-diffraction-limited focusing and efficient femtosecond continuum generation are demonstrated.

Femtosecond white-light continuum pulses.

We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time resolution of 80 fsec. These pulses extend continuously from 0.19 to 1.6 microm and have time sweeps as small as 10 fsec/1000 A. We find temporal, spatial, and spectral properties that are consistent with self-phase modulation having a prominent role in generation of the continuum.

Subpicosecond and picosecond studies of electron transfer intermediates in Rhodopseudomonas sphaeroides reaction centers.

The primary electron transfer processes in isolated reaction centers of Rhodopseudomonas sphaeroides have been investigated with subpicosecond and picosecond spectroscopic techniques. Spectra and kinetics of the absorbance changes following excitation with 0.7-ps 610-nm pulses, absorbed predominantly by bacteriochlorophyll (BChl), indicate that the radical pair state P+BPh-, in which an electron has been transferred from the BChl dimer (P) to a bacteriopheophytin (BPh), is formed with a time constant no greater than 4 ps. The initial absorbance changes also reveal an earlier state, which could be an excited singlet state, or a P+BChl- radical pair. The bleaching at 870 nm produced by 7 ps excitation at 530 nm (absorbed by BPh) or at 600 nm (absorbed predominantly by BChl) shows no resolvable delay with respect to standard compounds in solution, suggesting that the time for energy transfer from BPh to P is less than 7 ps. However, the bleaching in the BPh band at 545 nm following 7-ps 600-nm excitation, exhibits an 8- to 10-ps lag with respect to standard compounds. This finding is qualitatively similar to the 35-ps delay previously observed at 760 nm by Shuvalov at al. (Shuvalov, V.A., Klevanik, A.V., Sharkov, A.V., Matveetz, Y.A. and Kryukov, P.G. (1978) FEBS Lett. 91, 135-139) when 25-ps 880-nm excitation flashes were used. A delay in the bleaching approximately equal to the width of the excitation flash can be explained in terms of the opposing effects of bleaching due to the reduction of BPh, and absorbance increases due to short-lived excited states (probably of BChl) that turn over rapidly during the flash. The decay of the initial bleaching at 800 nm produced by 7-ps 530- or 600-nm excitation flashes shows a fast component with a 30-ps time constant, in addition to a slower component having the 200-ps kinetics expected for the decay of P+BPh-. the dependence on excitation intensity of the absorbance changes due to the 30-p]s component indicate that the quantum yield of the state responsible for this step is lower than that observed for the primary electron transfer reactions. This suggests that at least part of the transient bleaching at 800 nm is due to a secondary process, possibly caused by excitation with an excessive number of photons. If the 800-nm absorbing BChl (B) acts as an intermediate electron carrier in the primary photochemical reaction, electron transfer between B and the BPh must have a time constant no greater than 4 ps.