Grigory Adamovsky

Perspectives on the Interaction of Plasmas With Liquid Water for Water Purification

Plasma production or plasma injection in liquid water affords one the opportunity to nonthermally inject advanced oxidation processes into water for the purpose of purification or chemical processing. Such technology could potentially revolutionize the treatment of drinking water, as well as current methods of chemical processing through the elimination of physical catalysts. Presented here is an overview of current water treatment technology, its limitations, and the future, which may feature plasma-based advanced oxidation techniques. As such, this field represents an emerging and active area of research. The role that plasma-driven water chemistry can play in addressing emerging threats to the water supply is discussed using case study examples. Limitations of conventional plasma injection approaches include limited throughput capacity, electrode erosion, and reduced process volume. At the University of Michigan, we are investigating two potential approaches designed to circumvent such issues. These include direct plasma injection using an underwater DBD plasma jet and the direct production of plasmas in isolated underwater bubbles via a pulsed electric field. These approaches are presented here, along with the results.

Light-driven actuators based on polymer films

We describe new light-driven actuators based on films of polymer polyvinylidene fluoride, known as PVDF. The actuators employ the photomechanic bending of the polymer film caused by low-power (10 mW and less) laser radiation. The photomechanic effect combines various physical mechanisms, such as thermal expansion, the converse piezoelectric mechanism, and the photovoltaic and pyroelectric mechanisms, while the mechanism of thermal expansion is dominant. The force applied by the actuators to external objects is measured with a torsion balance. It is proportional to the power of laser beam and could be as high as 10–4 N for a 50-µm film illuminated with a 10-mW laser beam. We demonstrate mechanical vibrations of a 1×7-mm strip actuator at a frequency of 0.3 kHz. As examples of possible applications, a photonic switch and an actuator with a closed-loop motion that could drive the inner workings of a conventional mechanical clock were demonstrated. The proposed actuators have a potential of being used as propulsion components of future light-driven micro/nano systems.

Photomechanical effect in films of polyvinylidene fluoride

Photothermal bending of strips of polyvinylidene fluoride was initiated by a laser beam with a power of a few milliwatts. A bending strip generated a force of 10−4N that propelled a 1 g oscillating wheel of a mechanical clock. The frequency of photomechanical resonance at pulsed illumination was inversely proportional to the length of the strip. The proposed model explained bending as a result of uneven thermal expansion on opposite sides of the strip. The model predicted, in agreement with experiment, that the force is proportional to beam power and does not depend on the shape or position of the beam in the strip.

Holographic surface gratings in iron-doped lithium niobate

Surface gratings associated with holographic volume gratings in photorefractive crystals of iron-doped lithium niobate have been studied using diffraction of a reflected probe beam and high-resolution phase-shifted interferometric profilometry. Both techniques show that the surface gratings exist in the form of periodical corrugations of the same period as that of the volume grating. The maximum amplitude of the periodical surface relief measured by both techniques is close to 6.5 nm. We also demonstrated that the periodical electric forces on the surface were capable of assembling polystyrene microspheres along the fringes of the grating. Large amplitude of the periodic electric field (1.6×104 V/cm) is associated with the photogalvanic effect.

Fiber-optic thermometer using temperature dependent absorption, broadband detection, and time domain referencing

A fiber-optic thermometer based on temperature dependent absorption in Nd3+ doped glass is demonstrated over the 298–573 K range. A broadband detection technique allows the use of the complete spectrum of a pulse modulated light emitting diode (LED). A fiber-optic recirculating loop is employed to construct a reference channel in the time domain by generating a train of pulses from one initial pulse. A theoretical model is developed, and experimental data are shown to compare well with the theory. Possible sources of error and instability are identified, and ways to enhance the performance of the system are proposed.

Fiber-optic displacement sensor with temporally separated signal and reference channels

A fiber-optic displacement sensor with temporally separated signal and reference channels is proposed. The sensing technique used is based on determining the relative amplitude of the signal and reference pulses that together form a double pulse. The relative amplitude of the pulses in the double pulse is a function of the displacement. A setup to generate the double pulse with individual pulses of 5-ns duration and 10-ns delay is described. A novel signal processing technique, used to determine the relative amplitude of such pulses in the double pulse by analyzing different portions of the signal spectrum, is explained. Experimental data are also provided.

Performance Evaluation of Fiber Bragg Gratings at Elevated Temperatures

The development of integrated fiber optic sensors for smart propulsion systems demands that the sensors be able to perform in extreme environments. In order to use fiber optic sensors effectively in an extreme environment one must have a thorough understanding of the sensor’s limits and how it responds under various environmental conditions. The sensor evaluation currently involves examining the performance of fiber Bragg gratings at elevated temperatures. Fiber Bragg gratings (FBG) are periodic variations of the refractive index of an optical fiber. These periodic variations allow the FBG to act as an embedded optical filter passing the majority of light propagating through a fiber while reflecting back a narrow band of the incident light. The peak reflected wavelength of the FBG is known as the Bragg wavelength. Since the period and width of the refractive index variation in the fiber determines the wavelengths that are transmitted and reflected by the grating, any force acting on the fiber that alters the physical structure of the grating will change what wavelengths are transmitted and what wavelengths are reflected by the grating. Both thermal and mechanical forces acting on the grating will alter its physical characteristics allowing the FBG sensor to detect both temperature variations and physical stresses, strain, placed upon it. This ability to sense multiple physical forces makes the FBG a versatile sensor. This paper reports on test results of the performance of FBGs at elevated temperatures. The gratings looked at thus far have been either embedded in polymer matrix materials or freestanding with the primary focus of this paper being on the freestanding FBGs. Throughout the evaluation process, various parameters of the FBGs performance were monitored and recorded. These parameters include the peak Bragg wavelength, the power of the Bragg wavelength, and total power returned by the FBG. Several test samples were subjected to identical test conditions to allow for statistical analysis of the data. Test procedures, calibrations, and referencing techniques are presented in the paper along with directions for future research.

Dark spatial solitons in photopolymer films for optical interconnections

We demonstrate theoretically and experimentally that an initially Gaussian red light beam sent through a ?step phase mask and launched into a thin film of polymer poly(methyl methacrylate) doped with laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)4H- pyran known as DCM evolves into a spatial structure similar to a dark spatial soliton. This takes place due to the Kerr-type time-delayed nonlinearity associated with the mechanism of up-converted photobleaching of the dye-doped polymer. The proposed theoretical model is nonlocal in time and is based on the Shrodinger-type nonlinear equation for the amplitude of the propagating beam complemented by the rate equation for the light-induced decrease of the refractive index. The result of the structuring of the beam is the formation of a permanent pattern of the refractive index of the film that acts as a channel waveguide, trapping a weak Gaussian probe beam close to it. The probe beam can propagate along or against the soliton. We also demonstrate the tolerance of trapping to a possible shift or tilt of the probe beam with respect to the soliton. This makes the proposed approach potentially useful for interconnections between individual fibers, fiber ribbons, bundles, and multicore fibers as well as between fibers and planar integrated optical devices.

Thermal Evaluation of Fiber Bragg Gratings at Extreme Temperatures

The development of integrated fiber optic sensors for use in aerospace health monitoring systems demands that the sensors be able to perform in extreme environments. In order to use fiber optic sensors effectively in an extreme environment one must have a thorough understanding of the sensors capabilities, limitations, and performance under extreme environmental conditions. This paper reports on our current sensor evaluation examining the performance of freestanding fiber Bragg gratings (FBG) at extreme temperatures. While the ability of FBGs to survive at extreme temperatures has been established, their performance and long term survivability is not well documented. At extreme temperatures the grating structure would be expected to dissipate, degrading the sensors performance and eventually ceasing to return a detectable signal. The fiber jacket will dissipate leaving a brittle, unprotected fiber. For FBGs to be used in aerospace systems their performance and limitations need to be thoroughly understood at extreme temperatures. As the limits of the FBGs performance are pushed the long term survivability and performance of the sensor comes into question. We will not only examine the ability of FBGs to survive extreme temperatures but also look at their performance during many thermal cycles. This paper reports on test results of the performance of thermal cycling commercially available FBGs, at temperatures up to 1000 C, seen in aerospace applications. Additionally this paper will report on the performance of commercially available FBGs held at 1000 C for hundreds of hours. Throughout the evaluation process, various parameters of the FBGs performance were monitored and recorded. Several test samples were subjected to identical test conditions to allow for statistical analysis of the data. Test procedures, calibrations, referencing techniques, performance data, and interpretations and explanations of results are presented in the paper along with directions for future research.

Formation of a graded-index waveguide in UV exposed polyimide

We report on the formation of a graded-index waveguide in a photocrosslinkable fluorinated polyimide upon its photodarkening with the spectral line 313 nm of a mercury arc lamp at exposures exceeding 5 J/cm2. Photodarkening prevents the UV radiation from penetration deep into the polymer and finally sets the region with increased refractive index near the top surface. A theoretical model based on this concept reconstructs the refractive index profile of the waveguide in agreement with the phase integral calculations based on waveguide mode spectrum.