Paul R. Ashley

Amorphous silicon photoconductor in a liquid crystal spatial light modulator

An amorphous silicon photoconductor has been demonstrated in a reflection mode nematic field effect liquid crystal spatial light modulator. The thin-film photoconductor provided high resolution of >35 lp/mm and sensitivity better than 20 microW/cm(2). In addition the switching speed was liquid crystal limited. Reported are the performance characteristics as well as a theoretical model for the device.

Multilevel registered polymeric Mach–Zehnder intensity modulator array

Described here is the first known demonstration of a registered two level guided wave polymeric electro‐optic Mach–Zehnder intensity modulator array. The device consists of two complete vertically stacked levels. Both levels were independently poled and operated. There was no measurable optical or electrical cross talk due to a high resistivity thermoset polymer buffers layer employed. Fabrication and performance of the device is discussed.

Holographic Bragg grating input–output couplers for polymer waveguides at an 850-nm wavelength

Bragg gratings used as input-output couplers in polymeric waveguides have been demonstrated at infrared wavelengths. These Bragg grating couplers were holographically formed volume phase gratings with a near-45 degrees fringe slant angle embedded directly into a waveguide layer. A photopolymer was used for both producing a planar waveguide and constructing the embedded Bragg grating coupler. A coupling efficiency of 23% for input and 5% for output has been achieved at 850 nm. The output-coupling beam profiles are also discussed.

Fresnel lens in a thin‐film waveguide

The theoretical intensity profile of a phase shift and absorption Fresnel lens in a thin‐film waveguide has been calculated. An experimental lens has been constructed with F=5 and a focal length=4 mm using CeO deposited on a BaO waveguide. A 3‐dB spot size (full width half‐maximum) of 3 μm was obtained compared to a diffraction‐limited spot size of 2 μm from theory. Efficiency is approximately 25% and a variation of less than 3 dB in peak intensity is observed over incident angles of up to ±15° from normal.

Liquid crystal spatial light modulator with a transmissive amorphous silicon photoconductor

A spatial light modulator with a thin (1-microm) amorphous silicon (PIN) photoconductor has been demonstrated with a threshold sensitivity of <3 microw/cm(2). A novel compound electrode design greatly increases the efficiency allowing the use of a photoconductive layer thin enough to achieve good transmission. The performance characteristics are reported and compared to the predictions of a theoretical model of the device.

Channel waveguides in electro‐optic polymers using a photopolymer cladding technique

Reported here is a new fabrication technique for forming channel waveguides in organic electro‐optic (EO) polymer materials. The process uses projection printing to directly expose inverted ridge channel patterns in a UV curing optical epoxy cladding layer. This technique is noncontact, requires no post processing, minimizes wall roughness, and can be used with any spin coatable EO polymer. Single‐mode channel waveguides have been demonstrated and characterized as well as phase modulator devices.

INTEGRATION OF NONLINEAR OPTICAL POLYMER WAVEGUIDES WITH INGAAS P-I-N PHOTODIODES

Performance requirements for systems employing optical guided wave components may be satisfied more easily through the hybrid integration of nonlinear optical (NLO) polymers and semiconductors. Reported in this letter, is the integration of NLO polymer waveguides with InGaAs p‐i‐n photodetectors at 1.3 μm, with coupling efficiencies greater than 99% and system responsivities of 0.84 A/W. An analysis of the structure using a two‐dimensional beam propagation model was performed. Planarization was maintained and the design allows for further integration of optoelectronic components.

Measurement of nonlinear properties in Ag‐ion exchange waveguides using degenerate four‐wave mixing

Degenerate four‐wave mixing was used to evaluate third‐order nonlinearities in Ag‐Na exchange waveguides. Guided wave linear absorption analysis was used to correlate metallic Ag content with nonlinear properties. Measured conjugate reflectivities approached 4%.

Space application requirements for organic avionics

The NASA Marshall Space Flight Center is currently evaluating polymer based components for application in launch vehicle and propulsion system avionics systems. Organic polymers offer great advantages over inorganic corollaries. Unlike inorganics with crystalline structures defining their sensing characteristics, organic polymers can be engineered to provide varying degrees of sensitivity for various parameters including electro-optic response, second harmonic generation, and piezoelectric response. While great advantages in performance can be achieved with organic polymers, survivability in the operational environment is a key aspect for their practical application. The space environment in particular offers challenges that must be considered in the application of polymer based devices. These challenges include: long term thermal stability for long duration missions, extreme thermal cycling, space radiation tolerance, vacuum operation, low power operation, high operational reliability. Requirements for application of polymer based devices in space avionics systems will be presented and discussed in light of current polymer materials.

Design Considerations For Mode Extinction Modulators In LiNb0 3 Waveguides

Many types of electro-optic modulators have been considered for IOC applications. The mode extinction modulator is particularly interesting in that its operation is based on the composition of several physical mechanisms. These include coupling between guided modes, radiation to substrate modes, and voltage dependent propagation losses. Therefore, in addition to possessing the desired properties of high frequency response, low operating voltage, high efficiency, large packing density, and simplicity in fabrication, the modulators transfer curve can be altered to optimize a particular curve shape. This is accomplished by adjusting certain physical parameters in the waveguide structure or modulator electrodes. For example, a large linear dynamic range can be achieved by the proper choice of physical parameters. Optimization for high efficiency, linearity, and modulation depth will be reported along with design parameters and trade-offs. In addition, a technique for creating an optical bias will be discussed which allows operating point selection.