Spencer T. Cole

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.

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.

Solar Pumped Solid State Lasers for Space Solar Power: Experimental Path

We outline an experimentally based strategy designed to lead to solar pumped solid state laser oscillators useful for space solar power. Our method involves solar pumping a novel solid state gain element specifically designed to provide efficient conversion of sunlight in space to coherent laser light. Kilowatt and higher average power is sought from each gain element. Multiple such modular gain elements can be used to accumulate total average power of interest for power beaming in space, e.g., 100 kilowatts and more. Where desirable the high average power can also be produced as a train of pulses having high peak power (e.g., greater than 10(exp 10 watts). The modular nature of the basic gain element supports an experimental strategy in which the core technology can be validated by experiments on a single gain element. We propose to do this experimental validation both in terrestrial locations and also on a smaller scale in space. We describe a terrestrial experiment that includes diagnostics and the option of locating the laser beam path in vacuum environment. We describe a space based experiment designed to be compatible with the Japanese Experimental Module (JEM) on the International Space Station (ISS). We anticipate the gain elements will be based on low temperature (approx. 100 degrees Kelvin) operation of high thermal conductivity (k approx. 100 W/cm-K) diamond and sapphire (k approx. 4 W/cm-K). The basic gain element will be formed by sequences of thin alternating layers of diamond and Ti:sapphire with special attention given to the material interfaces. We anticipate this strategy will lead to a particularly simple, robust, and easily maintained low mass modelocked multi-element laser oscillator useful for space solar power.

Design strategies for ultrahigh peak power lasers in space

Summary form only given. We examine design issues for laser systems intended to operate in a space environment and also to achieve high average power and high peak power. Physical issues that impose novel demands include a need to remove heat by radiative processes; establish and maintain precise alignment of low mass structures extending over dimensions that can range to kilometers and more; draw all power from the conversion of sunlight to electrical power by solar cells; manage the distribution, with low mass technology, of substantial levels of heat, electrical power, and optical power over dimensions that can range to kilometers and more.

Electrically-Tunable Group Delays Using Quantum Wells in a Distributed Bragg Reflector

We present an optical delay line structure incorporating InxGa1-xAs quantum wells in the GaAs quarter- wave layers of a GaAs/AlAs distributed Bragg reflector. Applying an electric field across the quantum wells shifts and broadens the e1-hh1 exciton peak via the quantum- confined Stark effect. Resultant changes in the index of refraction thereby provide a means for altering the group delay of an incident laser pulse. Theoretical results predict tunable delays on the order of 50 fs for a 30-period structure incorporating 3 quantum wells per GaAs layer. Structure design, growth and fabrication are detailed. Preliminary group delay measurements on large-area samples with no applied bias are presented.