Richard L. Fork

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.

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.

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.

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.

Stretched Lens Array (SLA) for Collection and Conversion of Infrared Laser Light: 45% Efficiency Demonstrated for Near-Term 800 W/kg Space Power System

For the past 2frac12 years, our team has been developing a unique photovoltaic concentrator array for collection and conversion of infrared laser light. This laser-receiving array has evolved from the solar-receiving Stretched Lens Array (SLA). The laser-receiving version of SLA is being developed for space power applications when or where sunlight is not available (e.g., the eternally dark lunar polar craters). The laser-receiving SLA can efficiently collect and convert beamed laser power from orbiting spacecraft or other sources (e.g., solar-powered lasers on the permanently illuminated ridges of lunar polar craters). A dual-use version of SLA can produce power from sunlight during sunlit portions of the mission, and from beamed laser light during dark portions of the mission. SLA minimizes the cost and mass of photovoltaic cells by using gossamer-like Fresnel lenses to capture and focus incoming light (solar or laser) by a factor of 8.5X, thereby providing a cost-effective, ultra-light space power system

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.

Resonant transmissive modulator construction for use in beam steering arrays

We describe a design for an agile, electronically- configurable, optical beam steering array to be used in directional free-space transmission of optical signals. The proposed device employs a 1D array of tunable resonant transmissive modulators constructed from customized multi- layered stacks of dielectric materials. Each modulator may be individually configured to transmit an optical signal with a known amount of phase and group velocity modulation. Proper configuration of each individual modulator results in diffractive interactions between multiple modulator outputs, providing a method for directional optical signal transmission. Of particular focus within this paper is the design of the individual modulator. We generate custom transmission functions by varying the parameters describing the modulators specific construction, such as number of layers within the multi-layer stack, refractive indices of stack materials, layer thickness, and combinations of periodic versus non-periodic layer repetitions. A computational optimization of the variables describing the stacks construction strives to maximize the amount of optical signal modulation obtainable within defined limits. Our optimization is based largely on maximizing transmitted phase delay. We discuss trade-offs between methods of increasing device performance versus practical limitations of fabrication technologies.

Orbital Debris Mitigation Using Minimum Uncertainty Optical States [Point of View]

A large amount of space debris has already accumulated in near-Earth space at an alarming rate. If scientists and engineers still cannot figure out a way to clean up the accumulating junk in space, some of this debris will likely fall into the Earths atmosphere. This article discusses an option for cleaning up this space debris based on minimum uncertainty optical states. In this concept, these states are formed routinely as specific spatial and temporal distribution of light generated by optically adjusted lasers from the International Space Station (ISS), and is transmitted to a microsatellite as a minimum uncertainty spatial mode. The transmitted power is then converted to a train of minimum uncertainty space-time pulses at nearby microsatellite and is used to push space debris away from the ISS. However, the suggested experiment requires the installation of a state-of-the-art laser on the Kibo facility, which is now found on the ISS. This laser facility can provide adequate power and cooling capabilities, as well as a location for the laser, the optics, and the pointing and tracking required in performing the experiment. Though potential problems are expected to arise in implementing this experiment, the possibility of mitigating this orbital debris to keep the Earths path into space open is still good news. It will be up to engineers and scientists to use these tools wisely and in a timely manner.

Asteroid redirection using synchronized femtosecond pulse trains

N simulation of interaction between fluid flow and particle motion demands sophisticated algorithms due to the motion of particles and difficulty in creating the grid system. We developed, during past decades, numerical solution methods to tackle this problem and applied the methods to several branches of engineering applications of small scales. The method is based on the Lattice Boltzmann Method (LBM). In this presentation, we demonstrate three kinds of numerical solutions provided by the methods. First, we developed the simulation code for the problem of translocation of a biopolymer through a nano–pore driven by an external electric field. A theoretical formula is also used to calculate the net electrophoretic force acting on the part of the polymer residing inside the pore. Next, we simulated the motion of microscopic artificial swimmer. The swimmer consists of an artificial filament composed of super–paramagnetic beads connected by elastic linkers and an externally oscillating magnetic field is used to actuate the filament, and we have found that there is an optimum sperm number at which the filament swims with maximum velocity. Then, we computed the fluid flow generated inside a micro -channel by an array of beating elastic cilia. We have found that there exists a maximum flow rate at an optimum sperm number. We also simulated the motion of particles caused by fluid flow of cilia actuation.T Magnus effect is the phenomenon whereby a rotating body experiences an asymmetric force due to its rotation. Historically researchers (Benjamin Robins and Gustav Magnus) investigated this effect using spherical bodies. A simplified investigation later followed by limiting attention to two dimensions, reducing the sphere to a circle was performed. Potential flow theory was capable of describing this situation by superposing a uniform stream upon a collocated doublet/vortex flow. Integrating Euler’s equation along the surface of the resulting “rotating” circle yielded an asymmetric force. Experimental verification of this theoretical result was undertaken by approximating the two dimensional circle by a circular cylinder that spanned either a water or wind tunnel. Potential flow theory was taken by Ludwig Prandtl and expanded to describe the lifting flow about a three dimensional surface. Prandtl and his colleague Max Munk used this theory to derive the optimum distribution of vortex flow (hence, circulation) along the span of a lifting body. The elliptical distribution is the optimum in order to reduce induced drag. Given that optimum, Munk was able to solve for the optimum chord distribution for a fixed wing. The extension from two dimensional to three dimensional investigation for airfoils/fixed wings has outpaced that for rotating bodies. The majority of the work on rotating bodies to date has remained two dimensional. The author has taken the optimum circulation distribution and applied it to a rotating cylindrical body. The theoretically optimum three dimensional geometry has been derived and will herein be described.Like most accredited mechanical engineering programs, the undergraduate curriculum at California State University Chico includes a required course in Finite Element Analysis (FEA). Historically, the primary focus of the class has been the underlying theory of the method and its formulation from fundamental governing equations with little to no instruction in commercial software designed specifically for the purpose. Students were taught the traditional theoretical methods (Stiffness, Galerkin, Virtual Work, Castigliano, etc.) and were given assignment problems with rigorous hand-work such as assembling stiffness matrices. They were taught computer based solution methods through non-specific computational software such as Excel and MATLAB®. Feedback from advisory boards, capstone project sponsors, senior exit surveys, and other evidence clearly indicated a problem with the curriculum’s approach to finite element analysis. While program graduates were well versed in the theory of the method, there was strong evidence that they were not skilled its proper application via commercial FEA software, a very common task in the workplace. Observations included poorly posed problems, unnecessary computational rigor, meaningless results, or indeed the inability to obtain a solution at all. In response, the FEA course was redesigned to include basic instruction in the proper use of commercial FEA software while still maintaining sufficient theory for understanding the inherent assumptions and limitations of the method. Segments of theory-based discussion and traditional assignments are now followed with exploration of the same concepts in the context of commercial software. Emphasis is placed on its proper use, underlying assumptions, limitations, and validity of results.

Deflecting asteroids with femtosecond optical pulses

Recent findings suggest ‘rogue asteroids’ on a collision course with Earth are more abundant than previously thought.1, 2 Yet, at the time of writing, there is still no worldwide agreement regarding reliable means of preventing such collisions. Nuclear detonation might alter the course of such an asteroid, but useful methods to prevent collision will most likely include continuous monitoring and further optimization of the deflection process. Optical quantum energy in the form of energetic femtosecond optical pulses used in space offers such capabilities (see Figure 1). These pulses work by slowing down an asteroid for long enough, so that the Earth moving in its orbit passes unharmed through the location where the collision was otherwise expected to occur. The practical challenge is identifying and testing means of applying this optical quantum energy effectively. Energetic femtosecond optical pulses rapidly lose energy in the Earth’s atmosphere, much like a meteor. However, in the vacuum of space such pulses are not attenuated. Moreover, in a low-gravity (or microgravity) environment, the delivery of these pulses can be optimized to approach near-unit-efficient application of energy to deflection. In addition, the asteroid supplies the large majority of the propellant required for the deflection event. Assuming we can regard the asteroid as isolated in vacuum and microgravity, we see the practical challenges in using optical quantum energy to deflect asteroids mainly as generating a sufficient number of well-synchronized energetic femtosecond pulses per unit time, and applying these pulses optimally to the deflection of the threatening asteroid.3 The need for both maximally efficient deflection and a large number of sites for applying the deflecting ablative impulses favors what we term a ‘cooperative delivery strategy.’ Figure 2 shows a cross-sectional view of an asteroid illustrating the simultaneous use of two femtosecond optical pulses causing ablative propulsive thrust at two symmetrically positioned, locally Figure 1. Femtosecond pulse trains (green, yellow, light blue) cause ejecta (white cones) slowing an asteroid headed toward Earth. The thin white line represents the orbital path of the asteroid.