Shanalyn A. Kemme

Spectral identification of transverse lasing modes of multimode index-guided vertical-cavity surface-emitting lasers

The identification of lasing modes in vertical-cavity surface-emitting lasers (VCSELs) is usually based on the polarization-resolved near-field pattern identification, a method difficult to apply in the presence of multiple transverse modes. We study lasing modes of a multitransverse mode index-guided VCSEL under dc pumping conditions by considering in detail the modes’ spectral positions. The experimentally observed spectra are compared with theoretical calculations, based on a recently developed method for vectorial VCSEL eigenmodes [D. Burak and R. Binder, Appl. Phys. Lett. 72, 891 (1998)]. This allows for the identification of transverse lasing modes as a function of the pumping current. Two main effects that influence the spectral characteristics of the VCSEL are considered in our approach: temperature-induced linear shift of lasing wavelengths with increasing pumping current and switching of the lasing mode between different eigenmodes of the VCSEL cavity. The obtained temperature-dependent shift of...

Active guided-mode resonant subwavelength gratings.

We design and fabricate guided-mode resonant subwavelength gratings using an active layer of barium titanate. Loss mechanisms in the metal and in the guiding layer are investigated. Modeling and experimental results are shown.

Free-space modules and fiber array data links for optical interconnect systems

It is generally agreed that optics is advantageous for interconnect applications requiring high speed signal propagation over distances greater than several centimeters. This is especially true when signals must be transferred between substrates. Evidence of this exists in the development of high speed VCSEL based parallel data links. One approach to increasing the capacity of these systems is to use free-space optics to provide fanout, and fiber optic arrays to transfer signals over long connection distances. In this paper we summarize the transfer and interconnect properties of free-space optical systems used in conjunction with multi-mode parallel fiber arrays, and then discuss two designs for accomplishing fanout and fiber coupling functions.

Learning From Nature: Biomimetic Polarimetry for Imaging in Obscuring Environments

We find for infrared wavelengths there are broad ranges of particle sizes and refractive indices that represent fog and rain where the use of circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index systematically, we show that for specific scene parameters circular polarization outperforms linear polarization in maintaining the intended polarization state for large optical depths. This enhancement in circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Specifically, circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave infrared and the long-wave infrared, and large particle sizes of Sahara dust around the 4 micron wavelength.

Transmissive infrared frequency selective surfaces and infrared antennas : final report for LDRD 105749.

Plasmonic structures open up new opportunities in photonic devices, sometimes offering an alternate method to perform a function and sometimes offering capabilities not possible with standard optics. In this LDRD we successfully demonstrated metal coatings on optical surfaces that do not adversely affect the transmission of those surfaces at the design frequency. This technology could be applied as an RF noise blocking layer across an optical aperture or as a method to apply an electric field to an active electro-optic device without affecting optical performance. We also demonstrated thin optical absorbers using similar patterned surfaces. These infrared optical antennas show promise as a method to improve performance in mercury cadmium telluride detectors. Furthermore, these structures could be coupled with other components to lead to direct rectification of infrared radiation. This possibility leads to a new method for infrared detection and energy harvesting of infrared radiation.

Remotely Interrogated Passive Polarizing Dosimeter (RIPPeD).

Conductive polymers have become an extremely useful class of materials for many optical applications. We have developed an electrochemical growth method for depositing highly conductive ({approx}100 S/cm) polypyrrole. Additionally, we have adapted advanced fabrication methods for use with the polypyrrole resulting in gratings with submicron features. This conductive polymer micro-wire grid provides an optical polarizer with unique properties. When the polymer is exposed to ionizing radiation, its conductivity is affected and the polarization properties of the device, specifically the extinction ratio, change in a corresponding manner. This change in polarization properties can be determined by optically interrogating the device, possibly from a remote location. The result is a passive radiation-sensitive sensor with very low optical visibility. The ability to interrogate the device from a safe standoff distance provides a device useful in potentially dangerous environments. Also, the passive nature of the device make it applicable in applications where external power is not available. We will review the polymer deposition, fabrication methods and device design and modeling. The characterization of the polymers sensitivity to ionizing radiation and optical testing of infrared polarizers before and after irradiation will also be presented. These experimental results will highlight the usefulness of the conductive infrared polarizer to many security and monitoring applications.

Modal noise simulations due to high-frequency VCSEL numerical aperture fluctuations

We have experimentally identified and characterized two dynamic components of a VCSEL/multimode optical fiber link: (1) a high temporal frequency fluctuation of the VCSELs numerical aperture (NA) and, (2) source power spectrum shift in time (chirp) both upon ac modulation. We use these variables of the excitation irradiance distribution at a fiber input to simulate the resulting modal noise; without the typical assumptions that all fiber modes are equally filled and all relative fiber mode delays are equally probable. Results indicate that fluctuations in the NA can generate significant modal noise at the fiber output. On the other hand, source chip, in the presence of mode selective loss, is only moderately effective in generating modal noise. Moreover, since chirping increases with the VCSELs series resistance, its significance will diminish as design efforts to decrease the potential drop across the VCSEL are successful.

Scalar diffraction theory approach to estimating multimode-waveguide field-amplitude mode distributions.

We introduce a method to estimate the coupling coefficients of the guided field amplitude and the corresponding angular bandwidth in a multimode slab waveguide. This scalar diffraction theory approach is simpler than the more rigorous electromagnetic treatment and is directly applicable to communications systems that use large (dimensions or numerical aperture) waveguides, as in substrate-mode interconnects. Moreover, this method provides conceptual insight as to a parameters effect on the field-amplitude mode distribution and angular bandwidth.