Richard I. Billmers

Experimental demonstration of an excited-state Faraday filter operating at 532 nm.

We outline what is to our knowledge the first experimental demonstration of an excited-state Faraday filter. The filter consists of potassium vapor between crossed polarizers in a dc magnetic field and operates on the 4P((1/2)) ? 8S((1/2)) transition in potassium. The 4P((1/2)) state is populated by a linearly polarized, 10-ns light pulse from a dye laser operating at 769.9 nm. Another linearly polarized, 10-nsec pulse at 532.33 nm traverses the pumped volume of the K cell and is absorbed from the 4P((1/2)) state to the 8S((1/2)) state. The transmission of the filter is approximately 3.5% at 532.33 nm with a bandwidth of less than 10 GHz.

TEMPORAL CHARACTERISTICS OF NARROW-BAND OPTICAL FILTERS AND THEIR APPLICATION IN LIDAR SYSTEMS

The temporal characteristics of two different narrow-band optical filters at 532 nm are reported. Both filters operate on the 4P(1/2) - 8S(1/2) atomic transition of potassium vapor, where the 4P(1/2) state is excited by a 770-nm, 10-ns laser pulse. The filters operate on the principle of circular birefringence induced by either the Faraday effect or two-photon transition, which is confirmed by experiments. The characteristic decay times of the filters are 5 and 0.015 mus, which is significant for the design of optical gating and signal processing with matched filters for ocean lidar.

Holographic wavelength filters based on MEMPLEX(r) photopolymer

We present the design, development and characterization of volume holographic wavelength filters recorded in MEMPLEX - a new photoreactive polymer that has widespread applications in holographic technology. These holographic filters are characterized by their narrow wavelength and angular dephasing. MEMPLEX photopolymer exhibits complete lack of shrinkage during recording and post-processing, is completely self-supporting and can be fabricated to a size required by the application, has excellent optical quality, is resistant to most chemicals and has long shelf-life, it is therefore ideally suited for optical filter technology. We have recorded and characterized numerous holographic reflection volume gratings in 2 mm thick samples in order to examine the effect of writing geometry on wavelength and angle selectivity. The presented results show that efficient reflection gratings with more than 75% diffraction efficiency and linewidths of less than 0.2 nm can be recorded. Efficient wavelength filters can be used in many applications including telecom MUX/DMUX, LIDAR, spectroscopy, etc

Narrow-band holographic optical filter using thick efficient holographic gratings

RL Associates in conjunction with Hybrid Technologies is developing a narrow linewidth optical filter employing extremely thick volume holographic diffraction gratings. The gratings are written in MEMPLEX, a new holographic materia invented by Laser Photonics Technology, Inc. and licensed to Hybrid Technologies. MEMPLEX has the following characteristics: (1) Excellent optical clarity, (2) Preparable at any thickness up to 10 mm, (3) Large dynamic range for plane wave holograms, (4) Hard, freestanding, stable, polishable and coatable. We have written and characterized numerous gratings in 1.8 mm thick samples to study the effect of writing geometry on the spectral linewidth and field-of-view of a single grating in the reflection geometry. We have succeeded in writing some very efficient gratings at 15 degrees internal write angles with external slant angles of 5 degrees. These gratings exhibit linewidths of < 0.2 nm and diffraction efficiencies of better than 70 percent. The measured angular acceptance of these gratings ranges from 0.1 to 0.24 degrees. We have also written some initial angle multiplexed gratings which include 3 efficient gratings in the same volume in an attempt to increase the angular acceptance. In this manner we hope to achieve a highly efficient optical filter with extremely narrow spectral linewidth and wide angular acceptance. Filters based on thick volume holograms show great promise in Lidar applications and should result in superior S/N ratios.

FireLidar development: light scattering from wood smoke experiments and theory at 1574 nm

Measurements and lidar calculations have been made for 1574 nm laser light pulsed through hydrocarbon smoke generated by wood. A pulsed laser signal is directed to the end of wood smoke filled chamber. The signal is reflected back through the smoke by a mirror and the end of the chamber and the total returned energy is measured as a function of the smoke density. The results are compared with a lidar calculation using Rayleigh-Debye-Ganz scattering theory for fractal aggregates. Measurements and calculations are also made of the total backscattered signal for a smoke chamber with a non-reflecting surface. Relatively good agreement between the theory and experimental results are achieved in both cases. These results are used in the feasibility studies of a FireLidar active imaging system being developed for use in search and rescue in smoke and flame environments.

Development of FireLidar: an active imaging system for smoke and flame environments

FireLidar, an active optical imaging system, is being developed for use as an aid to search and rescue in smoke and flame environments. The system is intended to augment currently available passive thermal imaging technology by imaging in the presence of a thermal bloom, heavy smoke conditions, or species which strongly absorb thermal radiation, such as water. We present experimental verification of a theoretical model for FireLidar. Lidar range equations for compartment fire scenarios are derived and compared to measurements taken in a controlled smoke chamber. Extinction measurements of near-infrared light through soot particulate provide information about optical properties of fire environments necessary to predict Lidar returns. Measured extinction values are compared to a single-scattering approximation, based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Component specifications for a FireLidar prototype system are discussed, including laser power, filter bandwidth, and camera integration times. A man-portable prototype system using specified components is scheduled for completion by the end of 2005, with a handheld device following soon thereafter.

Modeling FireLidar: An active optical imaging system for smoke and flame environments

FireLidar, an imaging system for use in smoke and flame environments, offers many improvements upon prevalent thermal imaging technology. We demonstrate a model of the FireLidar system using Rayleigh-Debye-Gans scattering theory of polydisperse fractal aggregates. We are developing an affordable, man-portable, and eye-safe FireLidar prototype based on this model.

Evaluation of 8 x 8 reverse-etched avalanche photodiode arrays for oceanographic lidar systems

Testing has been conducted on 8 by 8 avalanche photo diode (APD) arrays derived from large area (16 mm) APDs, both produced by Advanced Photonics, Inc. The array structure was produced using a novel reverse etching process. Tests have been conducted measuring cross- talk, bandwidth, rise and fall times, gain, effective pixel size, and noise characteristics. Measurements have been made as functions of wavelength, optical intensity, and bias voltage. Cross-talk between pixels was characterized under both CW and pulsed (3 nsec) conditions. The effective pixel size was measured by scanning a very small laser spot (.25 mm) across the pixel under test while monitoring the output current. The measured pixel size was approximately 1 mm. This matched very well with the expected physical pixel size of 1 mm. The pulse response was measured by injecting a 3 nsec laser pulse into the pixel under test. The measured response shows that the signal decays approximately 3 orders of magnitude in 60 nsec. The rise time of the pixel is on the order of 5 nsec. Cross-talk between pixels was measured by injecting an optical signal into a pixel. The current output of an adjacent pixel was measrued as the optical power input was increased. The cross-talk a CW optical input is on the order of 1000 to 1. The pulsed cross-talk is on the order of 100 to 1. The cross talk ratio remains constant with varying optical input intensities. The pulsed wavelength response of the APD was characterized at 440 nm and 700 nm. The APD exhibited no difference between the two wavelengths.