Mary J. Potasek

Effects of chromatic dispersion and self-phase modulation in multihop multirate WDM rings

When compared to first generation and single-hop optical networks, multihop and multirate (M&M) networks have the advantage of significantly reducing network cost under a variety of wavelength-to-terminal cost ratios. This letter investigates how fiber chromatic dispersion and self-phase modulation may affect such cost reduction in M&M wavelength-division-multiplexing rings.

Generalized theoretical treatment and numerical method of time-resolved radially dependent laser pulses interacting with multiphoton absorbers

We introduce a generalized numerical method to calculate short-pulsed laser propagation in a wide class of multiphoton absorbing materials. The method has no restrictions on the input pulse widths varying from nanosecond to femtosecond, and its numerical solution is both radially and temporarily dependent, enabling us to check numerically the validity of assuming radially constant solutions, which ensures that the true peak intensity falls below the damage causing level. A new feature of our technique enables us to determine quantitatively the contributions to the total absorption due to every electronic energy level. We found excellent agreement between our calculations and experiments using sample materials ranging from reverse saturable absorbers, two-photon absorbers with excited-state absorption to three-photon absorbers. We applied our technique to a two-photon absorber with excited-state absorption and found approximately 1 order of magnitude increase in the absorption when femtosecond pulses were used in place of nanosecond pulses.

Multiterabit-per-second all-optical switching in a nonlinear directional coupler

In using numerical methods, we demonstrate all-optical soliton-like switching and self-routing of ultrashort (subpicosecond) optical pulses in a nonlinear directional coupler. As new modulation formats are introduced for high bit rate communications systems, all-optical switching techniques must be used that can operate at multiterabit-per-second rates.

Interstitial Photodynamic Therapy-A Focused Review.

Multiple clinical studies have shown that interstitial photodynamic therapy (I-PDT) is a promising modality in the treatment of locally-advanced cancerous tumors. However, the utilization of I-PDT has been limited to several centers. The objective of this focused review is to highlight the different approaches employed to administer I-PDT with photosensitizers that are either approved or in clinical studies for the treatment of prostate cancer, pancreatic cancer, head and neck cancer, and brain cancer. Our review suggests that I-PDT is a promising treatment in patients with large-volume or thick tumors. Image-based treatment planning and real-time dosimetry are required to optimize and further advance the utilization of I-PDT. In addition, pre- and post-imaging using computed tomography (CT) with contrast may be utilized to assess the response.

Highly effective thin film optical filter constructed of semiconductor quantum dot 3D arrays in an organic host

We calculate the imaginary part of the third order optical non-linearity for an array of semiconductor quantum dots in an organic host and show that it leads to large two-photon absorption. The calculated two-photon absorption is greater than currently measured materials. The large non-linearity results from a hybrid exciton formed in the inorganic-organic medium. The band gap of the semiconductor dot determines the spectral region of the resonances that vary from the visible to the near, mid and far infrared regions. We show that relatively small changes in the ratio of the quantum dot size to the quantum dot-to-dot spacing result in significant changes in the non-linearity. We briefly describe applications in communications, optical filters, and bio photonics for thin films comprising these hybrid excitions.

Experimental and numerical investigation of highly absorbing nonlinear organic chromophores

We have developed a mathematical/numerical framework based on computational transition modules and measured ultrafast laser light propagating through nonlinear materials. The numerical framework can be applied to a broad set of photo-activated materials and lasers, and can optimize photo-physical parameters in multi-photon absorbers. Two photon (TPA) processes are particularly useful in many applications including fluorescence imaging, optical data storage, micro-fabrication, and nanostructured quantum dots for optical limiters. Laser transmission measurements of the organic molecular chromophore, AF455-known TPA material-were taken with a 175 fs, λ0=780nm, plane-polarized light pulses from Ti:S regenerative amplifier into a 5.1mm thick PMMA slab doped with the chromophore. The range of input energies (intensities) in this experiment was 0.01μJ (0.97 GW/cm2) to 25 μJ (2.4 x103 GW/cm2). Experiments showed that for intensities beyond several μJ, the material did not saturate as predicted by traditional theory. We included excited-state absorption (ESA), as demonstrated by the absorption spectrum, which still could not account for the deviation. To understand this result we used our framework to show that an unexpected/unknown higher energy level was being populated. We calculated the entire experimental curve from 0.01μJ (0.97 GW/cm2) to 25 μJ (2.4 x103 GW/cm2) and found excellent agreement with the experimental data.

Numerical simulation of laser pulse propagation in rare-earth-doped materials

We describe a general numerical method for calculating short-pulse laser propagation in rare-earth-doped materials, which are very important as gain materials for solid-state lasers, fiber lasers and optical amplifiers. The split-step, finite difference method simultaneously calculates changes in the laser pulse as it propagates through the material and calculates the dynamic populations of the rare-earth energy levels at any position within the material and for times during and after the laser pulse has passed through the material. Many traditional theoretical and numerical analyses of laser pulse propagation involve approximations and assumptions that limit their applicability to a narrow range of problems. Our numerical method, however, is more comprehensive and includes the processes of single- and multi-photon absorption, excited state absorption (ESA), energy transfer, upconversion, stimulated emission, cross relaxation, radiative relaxation and non-radiative relaxation. In the models, the rare-earth dopants can have an arbitrary number of energy levels. We are able to calculate the electron population density of every electronic level as a function of, for example, pulse energy, dopant concentration and sample thickness. We compare our theoretical results to published experimental results for rare-earth ions such as Er3+, Yb3+, Tm3+ and Ho3+.

Self-routing of multi-terabit/sec pulses using an all-optical switch in a nonlinear coupler with dispersion flattened fiber

Using numerical methods we demonstrate all-optical switching and self-routing of ultrashort optical pulses in a nonlinear coupler. These ultrashort duration pulses may be suitable for operations in devices performing at multi-terabit/sec rates.

Light fluence dosimetry in lung-simulating cavities

Accurate light dosimery is critical to ensure consistent outcome for pleural photodynamic therapy (pPDT). Ellipsoid shaped cavities with different sizes surrounded by turbid medium are used to simulate the intracavity lung geometry. An isotropic light source is introduced and surrounded by turbid media. Direct measurements of light fluence rate were compared to Monte Carlo simulated values on the surface of the cavities for various optical properties. The primary component of the light was determined by measurements performed in air in the same geometry. The scattered component was found by submerging the air-filled cavity in scattering media (Intralipid) and absorbent media (ink). The light source was located centrally with the azimuthal angle, but placed in two locations (vertically centered and 2 cm below the center) for measurements. Light fluence rate was measured using isotropic detectors placed at various angles on the ellipsoid surface. The measurements and simulations show that the scattered dose is uniform along the surface of the intracavity ellipsoid geometries in turbid media. One can express the light fluence rate empirically as φ =4S/As*Rd/(1- Rd), where Rd is the diffuse reflectance, As is the surface area, and S is the source power. The measurements agree with this empirical formula to within an uncertainty of 10% for the range of optical properties studied. GPU voxel-based Monte-Carlo simulation is performed to compare with measured results. This empirical formula can be applied to arbitrary geometries, such as the pleural or intraperitoneal cavity.

Overview of computational simulations for PDT treatments based on optimal choice of singlet oxygen

Effective photodynamic therapy (PDT) treatment planning and treatment monitoring requires computer simulations of both light transport in tissue and photosensitizer (PS) photophysics to accurately estimate singlet oxygen. Simply using fixed prescribed values of light dose (fluence) or PDT dose (the time integral of ‘PS concentration’ times the ‘fluence rate’) – one value for all patients – does not account for differences in the amount of singlet oxygen formed when fluence rates change or patient tissue parameters change. We will focus on singlet oxygen dose which is calculated by solving the photokinetics rate equations and which determines the effectiveness of the subsequent reactions of singlet oxygen with the cancer target and the negative effect of PS photobleaching.