George A. Brost

Dispersion-enhanced photonic crystal fiber array for a true time-delay structured X-band phased array antenna

Tunable optical true time-delay modules based on highly dispersive photonic crystal fibers (PCFs) are demonstrated to provide continuous radio-frequency squint-free beam scanning for an X-band (8-12 GHz) phased array antenna system. The dispersion of the fabricated PCF is as high as -600 ps/nm /spl middot/ km at 1550 nm. The time delay is continuously tunable from -31 to 31 ps between adjacent delay lines by tuning the laser wavelength continuously from 1528 to 1560 nm. The far field radiation patterns of a 1/spl times/4 subarray were measured from -45/spl deg/ to 45/spl deg/ scanning angles. Squint-free operation is experimentally confirmed.

High-speed AJL8/APC polymer modulator

High-speed polymer modulators were fabricated using low-Vpi AJL8 chromophore in amorphous polycarbonate, and highly calibrated frequency response measurements were obtained using convenient coplanar-microstrip transitions. These Mach-Zehnder modulators show good frequency response to 50 GHz, with a loss and velocity mismatch-limited Vpi at 50 GHz <12 V. The measurements establish AJL8 as an excellent candidate for future military analog optical links

Radiation resistance of electro-optic polymer-based modulators

Mach–Zehnder interferometric electro-optic polymer modulators composed of highly nonlinear phenyltetraene bridge-type chromophores within an amorphous polycarbonate host matrix were investigated for their resistance to gamma rays and 25.6 MeV protons. No device failures were observed and the majority of irradiated modulators exhibited decreases in half-wave voltage and optical insertion losses compared to nonirradiated control samples undergoing aging processes. Irradiated device responses were attributed to scission, cross-linking, and free volume processes. The data suggests that strongly poled devices are less likely to de-pole under the influence of ionizing radiation.

Bandwidth study of volume holography in photorefractive InP:Fe for femtosecond pulse readout at 1.5 μm

The Bragg selectivity of volume holograms makes them not well suited for many Fourier imaging processing applications in the space domain because they perform the function of a spatial filter and limit the field of view. Similarly, for femtosecond pulse holography they reduce the spectral bandwidth of the diffracted signal. However, we show both theoretically and experimentally that it is much easier in the frequency domain than in the space domain to achieve a large enough diffraction bandwidth of volume holograms for the bandwidth of 100-fs pulses to be used for frequency-domain femtosecond pulse shaping. The experiments were performed by nondegenerate four-wave mixing in photorefractive InP:Fe with femtosecond readout at 1.5 μm.

Enhancement of beam coupling in the near infrared for tin hypothiodiphosphate

By using the moving grating technique, without an applied electric field, it is possible to significantly increase the steady‐state two‐beam coupling gain in photorefractive Sn2P2S6. Another technique of gain enhancement consists of cooling of the sample to −30u2009°C. The measured data confirm the existence of two out‐of‐phase gratings in Sn2P2S6 generated by charge carriers of different sign, with relaxation times of 70 ms and 500 s.

Observation of the photorefractive effect in vanadium-doped CdMnTe

Abstract We report observations of the photorefractive effect in vanadium-doped Cd0.55Mn0.45Te. The photorefractive response was observed in the red and near infrared, over the broad wavelength range of 0.633 to 1.32 μm. The photorefractive effects were characterized at 0.75 μm wavelength. We measure a grating formation time of 65 ms at an intensity of 20 mW/cm2. A Gain coefficient of 5.6 cm−1 was obtained with a 7 kV/cm ac electric field.

Nearly degenerate two-beam coupling in photorefractive crystals with two species of movable carriers

Two-beam intensity coupling is calculated for photorefractive crystals with two types of movable charge carrier in the undepleted-pump approximation. The analytical expressions are derived for the temporal evolution of the space-charge field; for weak coupling they are used for calculation of the transmitted beam intensities. The results of the calculation are compared with the experimental observations in photorefractive tin hypothiodiphosphate (Sn2P2S6). All experimental data are in reasonable quantitative agreement with the calculations.

True-time-delay modules based on a single tunable laser in conjunction with a waveguide hologram for phased array antenna application

A wavelength-controlled continuous beam-steering four-element X-band (8- to 12-GHz) phased array antenna system is presented. The system is based on the continuously tunable optical true-time-delay technique. Dispersion-enhanced waveguide holograms were proposed and used to fabricate the optical true-time-delay devices. The devices are characterized both theoretically and experimentally. The wavelength of a laser was tuned within the system to get continuously tunable true time delay. The time delay was measured for a wavelength tuning range from 1537 to 1547 nm in 10-nm steps. The far-field radiation patterns of the antenna system were measured at 9 and 10.3 GHz, and they showed no beam squint. The true-time-delay formation idea presented here is suitable for not only X-band, but also for higher microwave frequencies, such as K-band.

Behavior of NLO polymer modulators irradiated by gamma rays

Two second-order nonlinear optical chromophoric materials were investigated for their response to gamma-ray irradiations for doses ranging from approximately 10-104 krad(Si). Thin film polymer modulators composed of a mixture of amorphous polycarbonate and phenyltetraene [APC/CLD-1(CPW-1)] active regions with UV upper and lower UV claddings were investigated for their pre- and post-irradiation behavior. Modulator Vπ insertion loss, and extinction ratio responses were examined, while a blend of salmon deoxyribonucleic acid (DNA)- hexadeCetylTriMethylAmmonium Chloride (CTMA) film samples were studied for their spectral response following irradiations over the spectral range λ=240-2600 nm. Following irradiation ranging from 9.6-104 krad(Si), the DNA/CTMA films exhibited losses in transmissivity over the spectral range λ=882-2600 nm and increased transmissivity over portions of the 240 nm < λ < 882 nm band. Data from the study also suggested that strongly poled APC/CPW-1 modulators operating at λ=1550 nm and exhibiting low Vπ values were less likely to have their half-wave voltages affected by ionizing radiation. The optical insertion losses for the majority of the APC/CPW-1 irradiated mdolators were found to decrease following irradiation. Discussion of the experimental results and mechanisms believed responsible for the post-irradiation behavior and results are presented.

Radiation-resistant polymer-based photonics for space applications

Empirical data regarding the radiation induced responses of Mach Zehnder interferometric electro-optic polymer based modulators (PBMs) operating at 1310 and 1550 nm and broadband InP quantum dot (QD) polymer photodetectors (PPDs) operating into the near infrared (NIR) are reported. Modulators composed of spun-on materials and hybrid electostatically self assembled (ESA) and spun-on NLO materials are examined for changes to their half-wave voltage and insertion losses following a gamma-ray total dose of 163 krad(Si) and irradiation by 25.6 MeV protons at a fluence of ~1011 cm-2. Pre- and post- irradiation responses of ESA grown polymer detectors using InP QDs are examined for photovoltage degradation and aging effects. The data indicates and excellent potential for developing polymer based photonic (PBP) devices with increased radiation resistance suitable for transition to photonic space applications.