54,257 research outputs found
Self-collimated unstable resonator semiconductor laser
Self-collimation of the output is achieved in an unstable resonator semiconductor laser by providing a large concave mirror M sub 1 and a small convex mirror M sub 2 on opposite surfaces of a semiconductor body of a material having an effective index of refraction denoted by n, where the respective mirror radii R sub 1, R sub 2 and beam radii r sub 1, r sub 2 are chosen to satisfy a condition (R sub 2)/(1 + r sub 1) = (n - 1)/n, with a value of geometric magnification 1 less than or equal to M less than or equal to (n + 1)/(n - 1) where r sub 1 and r sub 2 are the radii of counterpropagating beams at respective mirrors of radii R sub 1 and R sub 2
Multiperiod-grating surface-emitting lasers
Surface-emitting distributed feedback (DFB) lasers are disclosed with hybrid gratings. A first-order grating is provided at one or both ends of the active region of the laser for retroreflection of light back into the active region, and a second-order or nonresonant grating is provided at the opposite end for coupling light out perpendicular to the surfaces of the laser or in some other selected direction. The gratings may be curved to focus light retroreflected into the active region and to focus light coupled out to a point. When so focused to a point, the DFB laser may be part of a monolithic read head for a laser recorded disk, or an optical coupler into an optical fiber
Evidence for a Critical Behavior in Pure Compact QED
We present evidence about a critical behavior of compact QED (CQED) pure
gauge theory. Regularizing the theory on lattices homotopic to a sphere, we
present evidence for a critical, i.e. second order like behavior at the
deconfinement phase transition for certain values of the coupling parameter
.Comment: 3 pages, 3 figures, POSTSCRIPT file (127KB uuencoded
Operations analysis (study 2.1). Program SEPSIM (solar electric propulsion stage simulation)
Program SEPSIM is a FORTRAN program which performs deployment, servicing, and retrieval missions to synchronous equatorial orbit using a space tug with a continuous low thrust upper stage known as a solar electric propulsion stage (SEPS). The SEPS ferries payloads back and forth between an intermediate orbit and synchronous orbit, and performs the necessary servicing maneuvers in synchronous orbit. The tug carries payloads between the orbiter and the intermediate orbit, deploys fully fueled SEPS vehicles, and retrieves exhausted SEPS vehicles when, and if, required. The program is presently contained in subroutine form in the Logistical On-orbit VEhicle Servicing (LOVES) Program, but can also be run independently with the addition of a simple driver program
Does offering more Advanced Placement courses increase enrollment?
This study utilizes a grant in California that required a group of high schools to increase the number of Advanced Placement (AP) courses offered to their students. The grant provides an arguably exogenous increase in the number of AP courses offered in a school. Using an instrumental variable approach, this analysis shows that offering an additional AP course does not increase total enrollment in AP courses. Instead, students substitute out of other AP subjects to enroll in the new subject being offered. This result suggests that additional AP course access is unlikely to induce students to enroll in more AP courses.Education, Advanced Courses, Education Policy
Tunable graphene bandgaps from superstrate mediated interactions
A theory is presented for the strong enhancement of graphene-on-substrate
bandgaps by attractive interactions mediated through phonons in a polarizable
superstrate. It is demonstrated that gaps of up to 1eV can be formed for
experimentally achievable values of electron-phonon coupling and phonon
frequency. Gap enhancements range between 1 and 4, indicating possible benefits
to graphene electronics through greater bandgap control for digital
applications, lasers, LEDs and photovoltaics through the relatively simple
application of polarizable materials such as SiO2 and Si3N4.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Semiclassical theory of noise in multielement semiconductor lasers
We present a derivation of the noise spectra of multielement semiconductor lasers. We model the noise by a set of Langevin sources which drive a system of small-signal field equations. The Langevin sources are normalized to transition rates within the laser and general formulas for relative intensity, frequency fluctuation, and field spectra are produced. We evaluate the formulas for several specific cases of interest, including those of a passive-active resonator and active-active coupled cavity resonator. In each case, the linewidth is governed by effective α-parameter(s) which generally differ from the material parameter. In the active-active cavity, the linewidth consists of two parts, one which is similar to the Schawlow-Townes linewidth, and a second which is proportional to the FM modulation index
- …